Chemistry
Chemistry
10th Edition
ISBN: 9781305957404
Author: Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher: Cengage Learning
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1. (McQuarrie 8-12) Use the variational method to calculate the ground-state energy of a particle
constrained to move within the region 0 ≤ x ≤ a in a potential given by:
V(x)
=
Vox
Vo(ax) ≤ x ≤ a
where Vo is a constant. Use a linear combination of the first two particle-in-a-box wavefunc-
tions as a trial function:
(x) = c1|1) + C2|42) = C₁
Пх
Σπα
sin
+ C₂
sin
a
a
a
a
Hint: Use the following integrals to simplify the calculus:
H11 =
(V₁| Ĥ| V₁) =
h²
8ma²
+ aVo
1
-+
H12 =
H21
=
(V1|Ĥ|42) = 0
(V2|Ĥ|41) = 0
H22
=
(V2|Ĥ| V2)
h² a Vo
+
2ma²
4
Note: These first two problems highlight how one can use choose to solve a given problem
with either the variational method or perturbation theory. In this class, you'll be told which
method to use, but in the real world, the quantum chemist needs to evaluate which method
is most appropriate for a given problem. That decision might be based on an understanding
of how big or small the perturbation is and how much effort the person is willing to apply to
solving the problem. Using a long, complicated trial function with many variational param-
eters will work well in most cases, but that can take a lot of effort to solve! In many cases,
using perturbation theory will be much easier, and it hopefully gives a reasonable result.
expand button
Transcribed Image Text:1. (McQuarrie 8-12) Use the variational method to calculate the ground-state energy of a particle constrained to move within the region 0 ≤ x ≤ a in a potential given by: V(x) = Vox Vo(ax) ≤ x ≤ a where Vo is a constant. Use a linear combination of the first two particle-in-a-box wavefunc- tions as a trial function: (x) = c1|1) + C2|42) = C₁ Пх Σπα sin + C₂ sin a a a a Hint: Use the following integrals to simplify the calculus: H11 = (V₁| Ĥ| V₁) = h² 8ma² + aVo 1 -+ H12 = H21 = (V1|Ĥ|42) = 0 (V2|Ĥ|41) = 0 H22 = (V2|Ĥ| V2) h² a Vo + 2ma² 4 Note: These first two problems highlight how one can use choose to solve a given problem with either the variational method or perturbation theory. In this class, you'll be told which method to use, but in the real world, the quantum chemist needs to evaluate which method is most appropriate for a given problem. That decision might be based on an understanding of how big or small the perturbation is and how much effort the person is willing to apply to solving the problem. Using a long, complicated trial function with many variational param- eters will work well in most cases, but that can take a lot of effort to solve! In many cases, using perturbation theory will be much easier, and it hopefully gives a reasonable result.
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