I. General

1. GAMESS input takes the form of a list of groups. Each group controls some aspect of the calculation, from the choice of basis set to the kind of calculation to the format for structure input.
   1. Each group has a name that begins with a $ sign, which must be in column 2 of the input line.

   2. After the options of the group are specified with a series of keywords, the group must be terminated with $END.

   3. The options within a group may be placed all on one line, or distributed over multiple lines. GAMESS recognizes the end of an input group by the $END.

2. The order of input groups below is a logical one, and its use is recommended.

3. Comment lines begin with a ! in column 1.

II. $CONTRL Group

1. A default GAMESS run is RHF/UHF for even/odd numbers of electrons, singlet state, a single point energy calculation, only the symmetry-unique atoms of the input structure specified, Cartesian coordinates in units of Angstroms. If these defaults are OK, then the $CONTRL group need not be given.

2. Following are some common options, specified by listing them following the $CONTRL.

   1. Optimize geometry: RUNTYP=OPTIMIZE

   2. ROHF calculation: SCFTYP=ROHF

   3. Locate transition state: RUNTYP=SADPOINT

   4. Calculate vibrational spectrum: RUNTYP=HESSIAN

   5. Do MP2 electron correlation: MPLEVEL=2

   6. Change multiplicity: MULT=n

   7. Charge on species: ICHARG=n (default, n=0, neutral)

   8. Input Cartesians for all atoms: COORD=CART

   9. Z-matrix (internal coordinates) input: COORD=ZMT

   Consult the Table of keywords for additional examples.

   (See subsequent section for instruction on Z-matrices)

3. A few examples of $CONTRL group input:
   1. Do ROHF calculation, optimize geometry, input is Z-matrix

      $CONTRL

      SCFTYP=ROHF RUNTYP=OPTIMIZE COORD=ZMT

      $END

   2. Do UHF calculation, all-atom Cartesian input, find transition state:

      $CONTRL

      SCFTYP=UHF, RUNTYP=SADPOINT, COORD=CART

      $END

III. The next group, $SYSTEM, is needed only occasionally. The principal use is to change the default time limit of 600.0 minutes:

$SYSTEM TIMLIM=900.0 $END

IV. Although the basis set may be specified as part of the $DATA group (below), it is more logically defined in its own group, $BASIS.

1. Ab initio basis sets

   Use GBASIS= to set the general type, and NGAUSS= to set the number of Gaussians. For example:

   $BASIS GBASIS=STO NGAUSS=3 $END

   sets the STO-3G basis.

   $BASIS GBASIS=N31 NGAUSS=6 $END

   sets the 6-31G basis.

2. To add diffuse functions to heavy atoms, include

   DIFFSP=.TRUE. [the periods are essential]

   To add a diffuse s shell to hydrogens, include

   DIFFS=.TRUE.

3. For polarization functions on hydrogen

   NPFUNC=n, where n = 1, 2, or 3

   Polarization on heavy atoms is set by

   POLAR=POPLE (the default)

   NDFUNC=n, where n = 1, 2, or 3

4. Semi-empirical basis sets are selected by

   GBASIS=MNDO, or AM1, or PM3

   Consult the Table for additional basis options.

V. $DATA is the first required input group.

1. The first four lines (referred to as cards in the documentation) consist of:
   • Line 1 = $DATA ($ in column 2)
   • Line 2 = a one-line title (your choice)
   • Line 3 = GROUP NAXIS (GROUP is the point group in the format CNV or DNH, NAXIS is the order of the principal axis)
   • Line 4 = usually blank

2. Line 5 and following lines depend upon your choice for COORD= in the $CONTROL group.
   1. If COORD=UNIQUE (the default), Line 5 and each succeeding line needed consist of:

      NAME ZNUC X,Y,Z

      where NAME is an atom identifier, up to 10 characters; ZNUC is the nuclear charge; and X,Y,Z are the Cartesians. These lines continue until all symmetry-unique atoms have been listed.

   2. If COORD=CART was chosen, Line 5 and succeeding lines have the same format as under B1, but one line is input for each atom. However, the Cartesians must reflect any symmetry specified in Line 3.

   3. If COORD=ZMT was selected, a standard Gaussian-style Z-matrix (see subsequent section) is input, one atom per line. A blank line follows the last atom. Then follows a line for each symbolic variable used in the Z-matrix, of the form:

      VARIABLE VALUE

   4. Regardless of which of the three above modes is used, the last line of the group is

      $END ($ in column 2)

VI. The next occasionally useful group is $SCF. The principal use of this group is to activate a direct scf calculation:

$SCF DIRSCF=.TRUE. $END [periods essential]

The default value of DIRSCF=.FALSE. causes integrals to be written to disk for later use. If SCFTYP=GVB was selected under $CONTROL, group $SCF must be used to provide GVB configurations.

VII. If an MP2 calculation has been selected under $CONTROL, the group $MP2 can be used to limit the calculation by coring out some occupied orbitals or omitting some virtuals:

NCORE=n - omits first n occupied orbitals

NOMIT=n - omits last n virtual orbitals

VIII. Finally, if one wishes to change the initial guess at orbitals from the default, one could do so under the group $GUESS.

Consult the Table of keywords for further examples.