Lecture 4 Partial differentiation (c) So Hirata, Department of Chemistry, University of Illinois at...
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Transcript of Lecture 4 Partial differentiation (c) So Hirata, Department of Chemistry, University of Illinois at...
Lecture 4Partial differentiation
(c) So Hirata, Department of Chemistry, University of Illinois at Urbana-Champaign. This material has been developed and made available online by work supported jointly by University of Illinois, the
National Science Foundation under Grant CHE-1118616 (CAREER), and the Camille & Henry Dreyfus Foundation, Inc. through the Camille Dreyfus Teacher-Scholar program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not
necessarily reflect the views of the sponsoring agencies.
Partial differentiation
In quantum chemistry, we often deal with a function of more than just one variables, for example, f(x,y,z).
When considering the derivatives of a multi-variable function, we must be aware of with respect to which variable we are differentiating the function.
is used to indicate the derivative is with respect to z, and x and y are held fixed in the differentiation.yxz
f
,
Partial differentiation
∂ (round d) indicates the partial differentiation.
Consider a function of space (x) and time (t) variables, f(x, t). Let the space variable also depend on time x = x(t). In this case, a partial derivative of f with respect to t is different from the exact derivative because
xtx t
f
dt
dx
x
f
t
f
dt
txdf
),(
Partial differentiation
Other than that, partial differentiation follows essentially the same rules as usual differentiation.
Is this true?
YES and NO - this is only true if the variables held fixed are identical in the left- and right- hand sides.
1y x
x y
Partial differentiation
Consider the change in function f(x,y,z) caused by an increase in x (y and z held fixed) and then in y (x and z held fixed).
The result would be the same if we increase y first and then x.
xy
f
yx
f
22
Time-dependent Schrödinger equation We use partial derivatives for operators. For example, the energy operator is,
We do not differentiate x, y, z dependent part of the wave function by t (see the simple wave in the previous lecture)
zyxtiE
,,
The Schrödinger equation
For one-dimension, it is
The kinetic energy operator comes from the classical to quantum conversion of the momentum operator
dx
dip
The Schrödinger equation In three-dimension, we have three Cartesian
components of a momentum:
Accordingly, the momentum operator is a vector operator:
(“del”) is a vector
zip
yip
xip zyx
;;
iz
iy
ix
ipppp zyx ,,),,(
zyx /,/,/
The Schrödinger equation
Kinetic energy in classical mechanics:
(The vector inner product is )
In quantum mechanics:
m
ppp
m
p zyx
22
2222
222zyx ppppp
2
2
2
2
2
222
2
22 zyxmm
The Schrödinger equation in spherical coordinates
Instead of Cartesian coordinates (x, y, z), it is sometimes more convenient to use spherical coordinates (r, θ, φ)
cos
sinsin
cossin
rz
ry
rx
The Schrödinger equation
EzyxV
mH ),,(
22
2
2
2
2
2
2
22
zyx
sin
sin
1
sin
1122
2
222
22
rrrr
The kinetic energy operator can be written in two ways – Cartesian or spherical.
Homework Challenge #2
Derive the spherical-coordinate expression of (the green panel) using the equations in blue and orange panels.
2
2
2
2
2
22
zyx
sin
sin
1
sin
1122
2
222
22
rrrr
cos
sinsin
cossin
rz
ry
rx
2