-stacking 
 in radical dimers , some of which are illustrated 
 in chart 1 in the form of the constituent monomers , 
 is responsible for the formation of a very interesting class of chemical 
 compounds which display favored packing geometries as described by 
 the maximum multicenter overlap principle between neighboring molecules .
this preferred orientation is primarily due to 
 the energy lowering of the singly occupied molecular orbital ( somo ) 
 of the radical as it overlaps with its neighbor .
somo 
 stabilization can be rationalized by the simple molecular orbital 
 ( mo ) diagram shown in chart 2a in which the 
 bonding highest occupied molecular orbital ( homo ) is doubly occupied . 

for example , for the prototypical phenalenyl ( phy , 1 ) 
 dimer 12 , the efficient  
 overlap provides the driving force for the stabilization of the dimer which is responsible for contact distances significantly 
 shorter and interaction energies larger than those for typical van 
 der waals ( vdw ) interactions .
the term  pancake bonding  has been suggested for 
 this type of bonding .
a major motivation in making 
 and understanding these -stacking pancake interactions originates 
 in the quest to make new molecular materials with high electrical 
 conductivity and for spintronics .
a crucial condition for the suitability of -stacked 
 molecules for such purposes is a strong overlap and thus a strong 
 interaction and a short intermolecular bonding distance between the 
 stacked subsystems .
high electrical conductivities have been achieved 
 for systems with various derivatives of 1 , 2 , and other -stacking materials .
however , there 
 is a strong need to push the limits of pancake interactions to even 
 shorter distances and stronger interactions in order to offer new 
 opportunities for materials design .
a13 form single - bonded pancake dimers ; 4 forms 
 double - bonded pancake dimers .
13 form single - bonded pancake dimers ; 4 forms 
 double - bonded pancake dimers . therefore , is 
 there a possibility to create even stronger pancake 
 bonds and more attractive interactions ?
in fact , the answer is yes 
 as the dimer orbital diagrams , shown in chart 2b or 2c , demonstrate .
they are based on a 
 monomer with a triplet or a singlet diradicaloid ground state with 
 a low - lying triplet state in the latter case in combination with an 
 antiaromatic electron count ( 8 ) and antiaromatic character .
if such a situation exists , a double pancake 
 bonding could arise as a four electron / multicenter ( 4e / mc ) bonding 
 interaction with a formal bond order pmo of 2 since two bonding orbitals are doubly occupied .
in general 
 we compute the bond order pmo at the mo 
 diagram level as1where nbind is the number 
 of electrons in the bonding orbitals and nanti is the number of electrons in antibonding 
 orbitals .
athe formal bond order according 
 to eq 1 is 1 for 12 , 22 , and 32 ( single - bonded pancake ) 
 and 2 for 42 ( double - bonded 
 pancake ) .
the formal bond order according 
 to eq 1 is 1 for 12 , 22 , and 32 ( single - bonded pancake ) 
 and 2 for 42 ( double - bonded 
 pancake ) .
the ( 4e / mc ) bond would lead to a 
 significant improvement in terms 
 of the interaction strength in contrast to the formal bond order of 
 1 for 12 as indicated by chart 2 .
antiaromatic compounds are usually characterized 
 by low stability which makes the search for appropriate candidates 
 for the double pancake bond difficult .
a good example is given in the cambridge structural 
 database , csd , in the form of the phenyl 
 derivative and the 4-chlorophenyl derivative of 1,3,2,4,6-dithiatriazine , 4 , 
 where the phenyl and 4-chlorophenyl have been replaced by h. it will 
 be compared to an analogous stable 7-electrons radical , 1,2,4,6-thiatriazine,3 , that forms a traditional 2e / mc 
 bonded pancake bond . 1,3,2,4,6-dithiatriazine ( 4 ) is 
 a neutral molecule with 8-electrons which forms a very short 
 pancake bonded dimer according to its crystal structure .
astates are designated as s 
 for singlet , t for triplet , and q for quintet .
states are designated as s 
 for singlet , t for triplet , and q for quintet .
can the 1,3,2,4,6-dithiatriazine dimer really be viewed as an example 
 for the double pancake bond , and , if so , what can we learn from it 
 for the construction of other and possibly better examples ? to answer 
 these questions from a theoretical point of view in a thorough way 
 one has to go well beyond the simple mo schemes presented so far . 

it is crucial to understand the subtle interplay of two kinds of electron 
 correlation effects which make these -stacking interactions 
 so challenging to understand and design . on one side
there is the 
 quasi - degeneracy of the homo and lumo calling for multireference methods 
 for an adequate description whereas on the other side dynamical correlation 
 effects are essential for the description of vdw interactions .
it is the purpose of this contribution to resolve the question 
 of the energetic feasibility of a double pancake bond using the high - level 
 multireference average quadratic coupled cluster , mr - aqcc , theory .
this level of theory provides an excellent approach 
 to the simultaneous treatment of static and dynamic electron correlation . 

it has been successfully used previously in interpreting the bonding 
 characteristics of the phenalenyl dimer and the tcne anion dimer , two prototypical examples of pancake bonding .
the multireference 
 starting point assures that the multiradical character is included 
 in the theory from the outset , and the approximate coupled cluster 
 level assures that the millions of configurations necessary for the 
 dispersion interaction are well accounted for .
complete active 
 space self - consistent field ( casscf ) and 
 multireference averaged coupled cluster mr - aqcc/6 - 311++g(2d,2p ) calculations 
 including full geometry optimizations were carried out on the  
 dimers 32 and 42 .
the electronic state configurations of these 
 two  dimers with c2v symmetry are illustrated in figure 1 .
the casscf(2,2 ) ( 32 ) and casscf(4,4 ) ( 42 ) calculations have been performed 
 using the bonding and antibonding orbitals of the somos in 3 and 4 as the active orbital space for 32 and 42  dimers , respectively .
molecular orbitals ( mos ) created by 
 the casscf method were used in the mr - aqcc calculations including 
 gradients with the same active orbital spaces as used in the casscf 
 calculations .
the total space of configuration state functions ( csfs ) 
 was constructed by applying single and double excitations from valence 
 orbitals to all virtual orbitals for all reference csfs and imposing 
 generalized interacting space restrictions .
the 1s core orbitals of the c , n , and s atoms and 2s and 2p orbitals 
 of the s atoms were frozen throughout all mr - aqcc calculations ( additionally , 
 eight low - lying occupied orbitals were frozen in 42 ) .
the analysis of the radical character of the 
 complexes was performed by analyzing ( i ) the natural orbitals ( nos ) 
 of the one - particle mr - aqcc density matrix and ( ii ) the effectively 
 unpaired density using the nonlinear formula of head - gordon .
the columbus suite of programs was used 
 for the mr - aqcc and casscf computations .
in addition to the single state casscf(4,4 ) approach for the singlet 
 and quintet states , state averaged casscf calculations have been performed 
 on the triplet state dominated by two main configurations : 1 = |a1b1a2b2| and 
 2= |b1a1b2a2| .
density functional theory ( dft ) was used to supplement the mr - aqcc 
 calculations for three candidate molecules that are promising for 
 double pancake bonding .
illustration of the bonding and antibonding combinations 
 of the 
 two and four somos for 32 ( a ) 
 and 42 ( b ) , respectively .
figures 2 and s1 compare the crystallographic data 
 for the two experimentally observed 
 derivatives of the double bonded pancake dimer 42 .
the phenyl and chlorophenyl substitution has 
 little effect on the geometry of the dithiatriazine core validating 
 the use of 42 as a good model 
 for these experimentally observed systems .
structures of two substituted 
 dithiatriazine ( hcn3s2)2  
 dimers indicate close similarity in 
 their structures .
these dimers were excised from their respective 
 crystal structures : the phenyl derivative ( 52 ) and the 4-chlorophenyl 
 derivative ( 62 ) are derivatives of 1,3,2,4,6-dithiatriazine , 4 . for the cationic dimers
the coulomb 
 energy was estimated by using 
 the following formula based on the approximate qi atomic charges .
we use qi values based on electrostatic potentials 
 ( esp ) following the hu  lu 
yang charge fitting method 
 ( hly scheme ) in eq 2(28)2 in which c is taken as the 
 reference coulomb energy at d = 
 10.0  . the interaction energy eint(d ) of the dimer with intermolecular separation d between the monomers is computed at the mr - aqcc level , 
 as the energy 
 of the complex with reference to the energy at a separation of d = 10.0  where the overlap is sufficiently close 
 to zero:3further computational details 
 are given in the supporting information section .
the separation 
 of the vdw and the attractive somo  somo interaction is essential 
 for the analysis of the interaction energy , eint(d ) .
somo interaction 
 ( esomo  somo ) and the van der waals 
 ( evdw ) term4the vdw 
 term includes dispersion , 
 pauli repulsion , and electrostatic interactions .
evdw is approximated by the interaction energy einths.5computed for the high - spin 
 ( hs ) state taken at the same distance d since in 
 this case bonding and antibonding interactions derived from the somo 
 orbitals approximately cancel and pmo = 
 0 ( eq 1 ) . according to chart 3 , for 42 the singlet ( s ) , triplet ( t ) , and quintet ( q ) states 
 contain double , single , and no pancake bond character with formal 
 bonders pmo equal to 2 , 1 , and 0 , respectively 
 ( eq 1 ) . the singlet states of 12 , 22 , and 32 all possess a bond 
 order p of 1 .
the somo  somo interaction term 
 for both the single and double pancake bond , respectively , is then 
 approximated as follows:6 ls labels the respective low spin state which 
 is only singlet for 12 , 22 , and 32 but can be singlet or triplet 
 for 42 .
the high - spin state is triplet in 
 the former case and quintet 
 in the latter .
a version of this approximation restricted to ls = 
 singlet and hs = triplet has been used by mota et al . for the analysis of the interaction energy of pancake bonded 
 dimers of 1 and 2 and has been recently 
 validated for both of these systems within the context of the mr - aqcc 
 level of theory .
one result , relevant for this study , was that the vdw term becomes 
 repulsive at the short contacts typical for pancake bonds .
according 
 to this analysis , the pancake contacts shorter than the typical vdw 
 distance result from the large negative ( bonding ) esomo  somo pancake interaction .
total energy minimization of the dimer 
 structures at the mr - aqcc level followed by rigid scans as a function 
 of the shortest sulfursulfur distance ( ds  s ) were performed for the -stacking pancake 
 dimers 32 and 42 .
the resulting interaction energies are presented 
 in figure 3a and 3b , 
 while the derived esomo  somo pancake 
 bonding energy terms based on eq 6 are shown 
 figure 3c .
potential energy scans for ( a ) the singlet 
 and triplet states of 32 and 
 ( b ) the singlet , triplet , and 
 quintet states of 42 .
somo 
 interaction energies are represented in ( c ) and are defined in the 
 inset according to eq 6 .
computations refer 
 to c2v symmetry using 
 an mr - aqcc/6 - 311++g(2d,2p ) level of theory .
the fully optimized geometries and the respective interaction 
 energies 
 are discussed first ; key data are collected in table 1 .
the singlet minimum of the single pancake bonded 32 shows an interaction energy of 7.0 
 kcal / mol at ds  s = 2.870  . 

this distance is much shorter than the vdw distance of 3.60  and is clearly indicative of pancake bonding . 

the experimentally observed contact for the dimers of the diphenyl 
 substituted 3 in the crystal is 2.677  ( csd refcode 
 cuvtao ) .
the agreement between computation 
 and experiment is good , given the missing steric repulsions due to 
 the phenyls in the model compound used and due to intermolecular interactions 
 in the crystal also not included in the calculations .
the interaction 
 energy for the triplet state of 32 at the equilibrium geometry of the singlet ( ds  s = 2.870  ) is repulsive with + 11.8 kcal / mol , 
 a value which is used to approximate evdw at this distance according to eq 5 .
somo 
 binding energy of 32 is 18.8 
 kcal / mol at the equilibrium geometry computed from eq 6 and represents a significant attraction . on the other hand , 
 the modest attraction of 1.8 kcal / mol at the minimum distance ds 
s of 4.0  for the triplet state 
 of 32 corresponds well to what 
 is expected of pure vdw interactions in terms of both the location 
 and depth of the minimum .
data for 12 are from 
 ref ( 19 ) and for 22 from ref ( 3 ) .
s , t , and q stand for singlet , triplet , 
 and quintet states , respectively .
c 
 contacts for 12 and 22 and s  s contacts for the rest of 
 the dimers
computed high - spin 
 state using the 
 singlet ground state geometry of the dimer .
turning 
 to the double pancake case of 42 , the singlet minimum shows a much larger interaction 
 energy of 27.7 kcal / mol at a remarkably short contact distance 
 of ds  s = 2.571  ( table 1 ) .
this distance is considerably shorter ( by 0.3 
  ) than in the radical dimer 32 discussed above and by more than 1  shorter than the 
 vdw distance of 3.60  .
s distance for the 
 dimers of the phenyl substituted 4 in the crystal is 
 2.529  ( average of two values from csd , refcode dessid ) .
the value in the isostructural 4-chlorophenyl 
 derivative dimer is 2.522  ( average of two values from csd , 
 refcode paflaj ) which is still much longer 
 than the typical single s
the agreement between computation and experiment 
 is very good , and again the differences are largely attributable to 
 steric repulsions due to the two phenyls ( not present in the computations ) 
 and to intermolecular interactions in the crystal .
the vdw interaction 
 energy computed from eq 5 using the quintet 
 at the equilibrium geometry of the singlet is repulsive with + 62.5 
 kcal / mol .
somo 
 interaction for the singlet with such a short distance must more than 
 overcome this repulsive term .
somo 
 pancake bonding energy according to eq 6 with 
 the intermolecular distance ds 
s is shown in figure 3c . at the equilibrium 
 geometry of the singlet of 42 it reaches the amazingly large attractive value of 90.2 
 kcal / mol .
for comparison , according to figure 3b and table 1 the modest attraction of 1.8 
 kcal / mol at the minimum d = 4.1  of the quintet 
 corresponds well to what is expected of purely vdw interactions . the triplet interaction energy curve for 42 in figure 3b shows an intermediate 
 behavior between that of the singlet and quintet with a minimum at 
 3.6  and an interaction energy of 2.9 kcal / mol .
the 
 triplet state is used as a tool to connect the single- and double - bonded 
 pancake interactions in 42 . the 
 bare somo  somo pancake bonding energy term for the triplet 
 of 42 ( figure 3c ) coincides remarkably well with that of the typical pancake 
 bonded dimer of 32 .
as has been 
 discussed in connection with chart 3 , in the 
 triplet state of 42 only one 
 pancake bond is left over as compared to the singlet , and therefore 
 it agrees well with the singlet of 32 , which also represents one pancake bond .
the dissociation 
 limit of the 42 dimer yields 
 three degenerate states ( s , t , and q ) .
they arise 
 from the coupling of the two triplet monomers as discussed in the supporting information section s.iv .
there is 
 also a lower energy singlet of the monomer that shows signs of a second 
 order jahn  teller symmetry breaking .
the extent and character of unpaired density of 
 the complexes were analyzed by the natural orbitals ( nos ) of the one - particle 
 mr - aqcc density matrix and the effectively unpaired electron density , which provides a measure for the separation of an electron pair 
 into different spatial regions .
the total number of effectively unpaired 
 electrons ( nu ) is computed with the following 
 formula:7where ni refers to the i - th natural 
 orbital occupancy and n to the number of natural 
 orbitals .
we have selected the nonlinear formula given in ref ( 32 ) since it reduces the relative 
 contributions of the ni values that are close to 0 or 2 diminishing the contributions from 
 dynamical correlation and thus highlighting only the truly open - shell 
 contributions of the radical centers .
the total number of effectively 
 unpaired electrons , nu , is displayed in 
 figure 4a as a function of the ss 
 contact distance ( ds  s ) for the 
 different multiplicities of 42 and the singlet state of 32 . at large separations nu values around 
 4.28 e
are obtained for 42 exceeding the expected 4.0 e because 
 at the mr - aqcc level , in addition to the two unpaired electrons on 
 each of the two monomers , dynamical correlations provide a slight 
 excess over 4.0 e. as the contact distance is reduced , 
 the singlet nu is dramatically reduced 
 as the electrons start to pair . at the equilibrium d = 2.571 
the pairing is still incomplete where nu = 0.57 e indicates a remaining limited 
 polyradical character . the triplet nu values of 32 are not shown ; they are essentially constant at 2.4 e , representing the two unpaired electrons plus some contributions 
 from dynamical correlation . at large separations
the singlet state 
 of 32 has the same number of 
 unpaired electrons as the triplet , which , however , in the former case 
 is substantially reduced as the two - electron pancake bond is being 
 formed . at the equilibrium geometry ( d = 2.870  ) 

the nu = 0.93 e value 
 indicates a still existent significant diradicaloid character , typical 
 for such single pancake bonds .
the qualitative difference 
 compared to the single - bonded pancake is that the double - bonded pancake 
 bond is substantially shorter , and the multiradical character is reduced . 

the triplet state of 42 retains 
 a high value of multiradical character of nu near 3.5 e at its minimum of d = 3.6  indicating that it is located intermediate between 
 the quintet and singlet in terms of electron pairing .
figure 4b displays the natural orbital occupation 
 numbers ( noons ) for the relevant frontier orbitals of the dimers 32 and 42 shown in figure 1 .
all noon values are 
 evolving toward 2.0 and 0.0 at a similar pace as d is reduced .
however , at the respective equilibrium distance of each 
 molecule these values differ significantly since the ds 
s distance is considerably smaller for the double - bonded 42 pancake as compared to the single - bonded 32 pancake .
( a ) total number of effectively 
 unpaired electrons ( nu ) of 32 and 42 and ( b ) occupation numbers of the 
 frontier nos of the singlet states of 32 and 42 as a function 
 of the separation distance ( ds  s ) .
the unpaired electron densities , 
 shown in figure 5 , indicate this difference 
 also : the radical character of 
 the double - bonded 42 pancake 
 is much smaller as compared to the single - bonded 32 case .
effectively unpaired electron density ( isovalue 
 0.002 au ) and atomic 
 contributions for the singlets of 32 and 42 .
nu is the number of effectively unpaired electrons given 
 in parentheses indicating stronger electron pairing in 42 compared to 32 . extending the simple integer bond 
 order pmo defined in eq 1 , we computed a more 
 detailed bond order pno based on the two 
 and four frontier orbital noons for 32 and 42 , respectively,8where nebo is the number of 
 electrons in bonding 
 orbitals , neabo is the number of electrons in the antibonding orbitals 
 based on the natural orbital occupancies for the two frontier orbitals 
 for 32 and four frontier orbitals 
 for 42 , respectively .
this reflects a major difference in the occupancies , 
 which mirror the much stronger and shorter pancake bonds in the -stacking 
 dimer 42 compared to those in 
 the radical dimer 32 .
based on these insights we have designed three new 
 double pancake bonded dimers each with an even number of -electrons . 

these systems that might exhibit double pancake bonding were obtained 
 by substituting se for s in 4 and substituting s for ch in 4 , respectively , arriving in both 
 cases at isoelectronic 8-electron rings .
a further example 
 is based on the c5h5 ring that has 
 a triplet ground state with d5h symmetry and exhibits an antiaromatic electron count .
computational results at the mr - aqcc and ub3lyp 
 levels indicate that these systems exhibit very short intermolecular 
 -stacking contacts as expected from double pancake bonding . the 
 optimized geometry of a hypothetical double pancake bonded dimer using 
 ub3lyp/6 - 311++g(2d,2p ) is shown in figure 6 .
the udft geometry optimization ( including ub3lyp and um06 - 2x ) on 
 the ( se2n3ch)2  dimer ( 72 ) using the 6 - 311++g(2d,2p ) basis 
 set provided strong evidence for double pancake bonding in this dimer . 

the udft methods produced a c2v symmetry optimized geometry for the ( se2n3ch)2 dimer with intermolecular se 
se distances 
 of 2.770  ( ub3lyp ) and 2.685  ( um06 - 2x ) .
these contact 
 values are strikingly shorter by 1.030  than the vdw distance 
 for sese ( 3.800  ) .
the rest of the intermolecular distances in the ( se2n3ch)2  dimer are similar to those of the 
 ( s2n3ch)2  dimer , 42 .
the ( se2n3ch)2  dimer also has a large interaction energy of 27.0 
 kcal / mol ( ub3lyp/6 - 311++g(2d,2p ) ) resulting from the perfect somo  somo 
 overlap , indicating that this unique se - bearing dimer displays strong 
 double pancake bonding and therefore would be a good candidate for 
 further analysis and perhaps synthesis .
the total interaction energy 
 is comparable to that in 42 : 
 27.0 indicating the overall strength of the double pancake 
 bonding and its ability to more than overcome vdw ( pauli ) repulsion 
 at these extremely short contact -stacking distances .
optimized geometry 
 of the se analogue of 42 with 
 ub3lyp/6 - 311++g(2d,2p ) .
two low - lying local minima of the hypothetical double pancake bonded 
 dimer , ( s3n3)2 ( 82 and 92 ) , were obtained by ub3lyp/6 - 311++g(2d,2p ) and are shown in 
 figure 7 .
the 82 configuration 
 with 
 pecfect overlap was further considered by rigid scan calculations 
 at the mr - aqcc(4,4)/6 - 311++g(2d,2p ) level starting with the ub3lyp/6 - 311++g(2d,2p ) 
 dimer optimized geometry . as shown in figure 8 ,
the total energy curve ( eint ) indicated 
 that there is a metastable minimum with a significant barrier to dissociation 
 into two s3n3 fragments , which arises 
 from strong cation 
the minimum 
 of this corrected interaction energy of the  dimer is 40.7 
 kcal / mol at about d = 2.7  .
furthermore , the 
 somo  somo interaction of 82 has been investigated by subtracting the interaction of the highest 
 spin state ( quintet ) from the interaction of the singlet state as 
 shown in figure 8b .
somo 
 component in the intermolecular interactions , which is very similar 
 to the behavior of 42 providing 
 a tremendeous driving force toward establishing a double bonded  
 stacking pancake . in the 82 case
the ss 
 contact was computed to be 0.812  shorter than the vdw distance 
 after subtracting a large coulomb repulsion term that was approximated 
 using esp based approximate point charges .
the total interaction energy 
 after subtraction of the coulomb repulsion is even larger than that 
 for 42 : 27.7 kcal / mol . 
 the respective esomosomo value ( 87.5 kcal / mol ) 
 and the short contacts indicate that the somo  somo bonding 
 interaction is very strong , and
potential energy scans of the singlet 
 and quintet of the ( s3n3)2  dimer with d3h symmetry ( 82 ) as a function 
 of the ss 
 distance ( ds  s ) computed at the 
 mr - aqcc(4,4)/6 - 311++g(2d,2p ) level .
c5h5 has the right electron count 
 to be a candidate for double pancake bonding .
we turned 
 to the perfluoro derivative of c5h5 , because we anticipated that the use of -electron withdrawing 
 groups will facilitate pancake bond formation .
the optimized geometry of the hypothetical double pancake 
 bonded -dimer , ( c5f5)2 ( 102 ) , using ub3lyp/6 - 311+g(d ) 
 is shown in figure 9 .
the dimer of 102 represents the first five - member ring forming a double 
 pancake bonded system with two perfectly degenerate somo 
thermodynamically , 102 is unfavorable because of ( i ) the strong coulomb repulsion 
 and ( ii ) the  dimer is much more favorable by means of cycloaddition . 

we suspect that the latter mechanism is a main reason why pancake 
 bonded systems with rings consisting of mostly c(sp ) have 
 not yet been charaterized .
energy minimization of the singlet ground 
 state of the dimer of the perfluoro derivative of c5h5 ( 10 , c5f5 ) showed a well - defined local minimum with overall positive 
 interaction energy due to the large coulomb repulsion .
these three 
 examples indicate that it should be possible to find further systems 
 that exhibit double pancake bonding .
it has been established for the first time through high - accuracy 
 quantum mechanical modeling that the -stacking dimer of 42 can be understood as a double pancake 
 bonded molecular aggregate .
this finding enriches the toolkit of chemical 
 interactions in a sensitive area connecting the weak vdw interactions 
 to electron pair chemical bonds .
the search for shorter and stronger 
 pancake bonding may lead to intermolecular contacts that might breach 
 into the range of extremely stretched single bonds in terms of 
 bond distance and binding energy .
the best candidates for utilizing 
 this new double pancake bonding mechanism will be likely found among 
 -electron rich molecules with their highest two occupied orbitals 
 being of -type concomitant with either a singlet ground state 
 with a low - lying triplet state and diradicaloid character or -electron 
 rich molecules with a triplet ground state .
 the 
 concept of a double - bonded pancake bonding mechanism is introduced 
 to explain the extremely short  stacking contacts 
 in dimers of dithiatriazines . 
 while ordinary single pancake bonds 
 occur between radicals and already display significantly shorter interatomic 
 distances in comparison to van der waals ( vdw ) contacts , the double - bonded 
 pancake dimer is based on diradicaloid or antiaromatic molecules and 
 exhibits even shorter and stronger intermolecular bonds that breach 
 into the range of extremely stretched single bonds in terms of bond 
 distances and binding energies . 
 these properties give rise to promising 
 possibilities in the design of new materials with high electrical 
 conductivity and for the field of spintronics . 
 the analysis of the 
 double pancake bond is based on cutting edge electron correlation 
 theory combining multireference ( nondynamical ) effects and dispersion 
 ( dynamical ) contributions in a balanced way providing accurate interaction 
 energies and distributions of unpaired spins . 
 it is also shown that 
 the present examples do not stand isolated but that similar mechanisms 
 operate in several analogous nonradical molecular systems to form 
 double - bonded -stacking pancake dimers . 
 we report on the amazing 
 properties of a new type of stacking interaction mechanism between 
  conjugated molecules in the form of a  
 double pancake 
 bond  which breaks the record for short intermolecular distances 
 and provides formidable strength for some  stacking 
 interactions . 
Introduction
Methods and Models
Results and Discussion
Conclusions