Computational Chemistry

Chapter 2, Harder Questions, Answers

Q10

Consider two PESs for the HCNÆHNC reaction: (a), a plot of energy versus the H–C
bond length, and (b), a plot of energy versus the HCN angle. Recalling that HNC is
the higher-energy species (Fig. 2.19), sketch qualitatively the diagrams a and b.

H C N

C N H

C N H

H

C N

r 2

r 1

r 3

a 1 = 0°

a 2

Angle exaggerated a little

H C N

r(H-C)

E E

a(HCN)

H

C N

a 3 = 180°

Chapter 3, Harder Questions, Answers

Q1

One big advantage of MM over other methods of calculating geometries and
relative energies is speed. Does it seem likely that continued increases in computer
speed could make MM obsolete?
Could increases in computer speed could make MM obsolete? The answer
would seem to be yes, eventually. If computer speed increases indefinitely, the
essentially complete solution of the Schr€odinger equation will become possible for
bigger and bigger molecules. This solution is the holy grail of computational
chemistry, as such a solution should accurately predict the properties of the
molecule. All computations might be perfectly accurate if computers were infinitely
fast, a probably unattainable goal, but one that might be effectively approximated
should practical quantum computers ever become available [1, 2].

References

1. Benenti G, Casati G (2005) Europhysics News 36:16


2. Scott A (2008) Scientific American 298:50

602 Answers