Concise Physical Chemistry

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

PROBLEMS AND EXAMPLES 341

$CONTRL SCFTYP=RHF MULT=1 RUNTYP=OPTIMIZE COORD=ZMT $END

$BASIS GBASIS=STO NGAUSS=2 $END

$DATA

Methanol

cs

C

O,1,R1

etc.

$END

FILE 20.13 Control lines for a GAMESS calculation.NGAUSScan be set to 2,3,...,6as

desired in line 2 of the control file. Az-matrix is selected here but either internal or external

coordinates can be used (see File 20.7).

Example 20.3 Symmetry Unique Atomic Coordinates

In some cases, symmetry can be used to reduce the demand on computer resources.

Although symmetry is most important in carrying out high-level calculations on large

molecules, the concept can be illustrated by a very simple example, the input file for

molecular hydrogen in the GAMESS procedure (Section 20.8.1). In this example, we

shall go from the GAMESS input file for molecular hydrogen in File 20.7 to one that

makes use of the plane of symmetry separating one hydrogen atom from an identical

partner at an equal distance from the center of symmetry.

Solution 20.3 The file presented in File 20.7 contains more information and de-

mands more computation than is necessary. Any calculation done on one H atom

must be identical to the calculation done on the other. Therefore, why not do one

calculation and be done with it?

This is accomplished by replacing the two-line Cartesian coordinate location

matrix by one line specifying the location of one of the two hydrogen atoms rel-

ative to their center of symmetry. In theCONTRLline,COORD=CARTis replaced

byCOORD=UNIQUEandDn 1is replaced byDnh 4.The distance of the se-

lected H atom from the center of symmetry is one half the bond length, 0.35A. ̊

If the bond length is not known,RUNTYP=OPTIMIZEcan be inserted before

COORD=UNIQUE.

$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZE COORD=UNIQUE $END

$BASIS GBASIS=STO NGAUSS=2 $END

$DATA

Hydrogen

Dnh 4

H 1.0 0.0 0.0 0.35

$END