## 1F: The z-matrix in the Gamess input file

The conventions used for the Gamess type of z-matrix are different from the MOPAC format.
MOPAC can generate such a z-matrix. The keyword AIGOUT produces a Gaussian type of input file, for which the z-matrix part is the same as for Gamess. The keywords and other directives have to be replaced.

The main differences are:
alpha-numeric symbols are used for variables,
the reference atoms appear in front of the symbols,
numeric values are interpreted as constants,
the z-matrix is followed by, optionally, a list of variables declared as constants, and a list of values for the variables in an optimisation.

An example:

```TITLE
title line entered here
ZMAT ANGS			indicates start z-matrix
C1				origin
C2  1  ccd			ccd = distance C2-C1
C3  2  ccs  1  a1		ccs=distance C3-C2, a1=angle C3-C2-C1
H1  1  ch   2  a1  3  0.0	torsion H1-C1-C2-C3 is 0.0
H2  1  ch   2  a1  3  180.0	Even with the directive RUNTYPE OPTIMISE
H3  2  ch   1  a1  4  180.0	these four torsions are kept constant
H4  3  ch   2  a2  1  180.0
H5  3  ch   2  a2  1  d1
H6  3  ch   2  a2  1  -d1	Note symmetry relation with previous torsion
[constants
follows list of constants, if any]
variables
ccd=1.34			list of variables
ccs=1.52
ch=1.09
a1=120.0
a2=109.5
d1=60.0
END				marks end of z-matrix
BASIS STO3G			specifies basis set
RUNTYPE OPTIMISE		type of calculation
VECTORS ATOMS			starting guess MO's
ENTER 1				closes input file
```
Note how symmetry relations are entered by using symbols more than once.

What is the structure of this molecule?
Are all symmetry relations valid?

Other valid basis sets are;
SV 3-21G, SV 4-31G, 6-31G*, TZVP, etc. See the Gamess manual for an overview.

Alternative RUNTYPE's are:

• SCF single point integrals and SCF calculation
• SADDLE search for saddle point; see chapter 2.
• FORCE force constant calculation at equilibrium geometry

This concludes Chapter 1.
Back to Contents of Chapter 1.
Next chapter How to locate a Transition State.