In this perspective, the various measures of electron
correlation
used in wave function theory, density functional theory and quantum
information theory are briefly reviewed. We then focus on a more traditional
metric based on dominant weights in the full configuration solution
and discuss its behavior with respect to the choice of theN-electron and the one-electron basis. The impact of symmetry
is discussed, and we emphasize that the distinction among determinants,
configuration state functions and configurations as reference functions
is useful because the latter incorporate spin-coupling into the reference
and should thus reduce the complexity of the wave function expansion.
The corresponding notions of single determinant, single spin-coupling
and single configuration wave functions are discussed and the effect
of orbital rotations on the multireference character is reviewed by
analyzing a simple model system. In molecular systems, the extent
of correlation effects should be limited by finite system size and
in most cases the appropriate choices of one-electron andN-electron bases should be able to incorporate these into
a low-complexity reference function, often a single configurational
one.