Concise Physical Chemistry

c20 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

340 QUANTUM MOLECULAR MODELING

using three parameters is denoted B3LYP. At present, B3LYP calculations are quite

satisfactory for smaller hydrocarbons but do not compete successfully in studies of

larger molecules.

PROBLEMS AND EXAMPLES

Example 20.1 Methane

Extend input File 20.7 to methane by including more basis functions inz-matrix

format. Use theoptkeyword in the route section of File 20.12 to find the optimized

bond length.

Solution 20.1 In File 20.12, the hydrogen atoms are added inz-matrix format

sequentially below the carbon atom C, designated atom 1, the reference point. For the

first H atom, we need to know only the distance relative to C,r= 1. 1 A. To locate ̊

the second hydrogen, we needrand the simple H C H angle, 109◦; and for the

third and fourth hydrogens we need the distance, the simple angle, and the dihedral

angles with the hydrogens already in place,− 120 ◦and 120◦.

# STO-2G opt

Methane

01

C

H,1,R

H,1,R,2,A

H,1,R,2,A,3,D1

H,1,R,2,A,3,D2

Variables:

R=1.1

A=109.

D1=-120.

D2=120.0

FILE 20.12 Az-matrix input file for methane. The distances and angles are estimates.

Example 20.2 A GAMESS Calculation of Methane

For those wishing to perform the calculation using GAMESS freeware, the input

file consists of three command lines, a symmetry specificationcsfollowed by the

z-matrix as already shown. Be sure to include the blank lines in theCONTROLfile

leave a blank line for the wordVariables:. GAMESS can be a little more cranky

than user friendly GAUSSIAN, but hey,...you can’t beat the price.