Welcome to the introduction to the quantum
chemistry playlist.
Quantum mechanics will be studied heavily
in this course, as opposed to classical mechanics.
Classical mechanics is the laws of physics
that are used to describe the behavior of
everyday-sized objects.
You're familiar with Newton's laws, how everyday-sized
objects like cars and boats and people move,
with the various forces that act on them.
That's all classical mechanics.
It's the kind of thing you learn about in
introductory general physics courses.
Quantum mechanics, by contrast, is the laws
of physics for very very small objects.
This is critical in chemistry because the
scale at which quantum mechanics becomes important
is the scale at which electrons, atoms, and
molecules exist.
To compare and contrast these two sets of
laws we have classical mechanics versus quantum
mechanics.
Classical mechanics describes objects that
are very large.
Quantum mechanics describes objects that are
very small.
CM describes things that are heavy, versus
QM describing things that are very very light:
electrons, atoms, molecules are all very light
compared to things like people, dumbbells,
anything that would have a mass that you could
reasonably measure in kilograms.
Anything you could touch, any macroscopic
object.
Classical mechanics is continuous, meaning
that you can have very very small changes
in all physical properties.
I can change the position, velocity, energy,
momentum, any property of whatever object
I'm interested in by very very small amounts.
By contrast in QM the behavior of objects
is discrete.
It is what you would call "quantized".
That's the root word of where "quantum" mechanics
comes from.
Discrete means that there are a finite number
of states that a particle can have.
It can't change its energy by any tiny amount,
it can only have certain "allowed" values
of energy.
For CM, you would solve Newton's equations,
F = ma (Newton's second law).
In QM you use the Schrodinger equation, which
we'll derive several videos down the line.
When you solve Newton's equations you get
a trajectory.
Your particle starts at a certain position
and momentum at an initial time, and solving
Newton's equations (F = ma) you get a trajectory
of where that particle moves over time.
Maybe it moves in a circle, maybe a straight
line, maybe it accelerates, but at any point
in time you know its position and momentum.
By contrast, in QM you get a wavefunction.
The Schrodinger equation gives you a wavefunction,
which is not a map of where the particle is
at any point in time, but describes the particle
as a wave.
The wave is spread out over space, and it
doesn't have well-defined values for its exact
position or momentum.
The only well-defined value will be the energy.
CM is what we call "deterministic".
If you start with the same initial conditions
(position, velocity), you'll get the same
result every time.
If you solve the equation again or do the
experiment again you should get the same result
within experimental error.
QM is what we would call "probabilistic".
If we have 2 systems in the exact same state
and we make a measurement for both, they might
not give the same result, but there are probabilities
for what result they will give.
Those probabilities for how it's going to
behave are given by the wavefunction.
The wavefunction tells us about the probability
for various possible outcomes, but it doesn't
tell us exactly what outcomes will occur in
the same way that a classical trajectory would.
CM is "intuitive".
We have an intrinsic understanding of how
CM works.
We're very familiar with how everyday objects
behave.
We know that if you drop a ball it will fall
to the Earth, accelerating while it does so.
We know how to throw things, how to catch
things, how to keep things from falling over.
We know how to build airplanes that fly, we
know how to build cars that turn the correct
direction.
We're familiar with how all of these objects
behave.
QM is usually non-intuitive to us, and has
a lot of features that you wouldn't have guessed
before learning about how the Schrodinger
equation and wavefunctions work.
That's because we're not familiar with how
things work at this very very small scale.
Things like atoms, electrons, and molecules
are going to behave differently than everyday-sized
objects.
That's because of this "quantum" behavior.
This quantum behavior comes into play at very
very small scales, and it's very different
than anything that we have experience with
in our everyday lives.
That is the basic intro to quantum chemistry.
The core of this course is ~120 videos organized
into ~10 chapters (links in the description).
They're organized into a course playlist as
well as various chapter playlists.
If you're looking for specific topics you
can look in the course playlist, or the links
in the description to the various chapters
and the topics within those chapters.
I'm trying to organize this channel to be
as easy to navigate as possible.
If you have trouble finding things you should
be able to search on the channel page, the
playlist pages, or even in the YouTube search
bar.
The results seem to be pretty good.
Let me know if you have any trouble.
Any comments you have I'm interested to hear.
Any feedback or suggestions you have you're
welcome to send in comments or by email as
well.
