Input¶

Input File¶

The following is a complete GAMESS input file for performing an RHF single-point energy calculation on a water (H₂O) molecule using the STO-3G basis set.

 $CONTRL COORD=CART UNITS=ANGS RUNTYP=ENERGY SCFTYP=RHF $END
 $BASIS GBASIS=STO NGAUSS=3 $END
 $SYSTEM MWORDS=200 $END
 $GUESS GUESS=HUCKEL $END

 $DATA
Water Molecule
C1
O      8.0      0.0000000000    0.0000000000    0.1068300000
H      1.0      0.0000000000    0.7851780000   -0.4273190000
H      1.0      0.0000000000   -0.7851780000   -0.4273190000
 $END

Explanation of Important Input Sections¶

A GAMESS input file is divided into several groups, each beginning with a $ sign and ending with $END. Every group controls a different aspect of the calculation.

1. `$CONTRL` Group¶

$CONTRL COORD=CART UNITS=ANGS RUNTYP=ENERGY SCFTYP=RHF $END

The $CONTRL group is the most important section of the input file. It specifies the type of calculation to be performed.

Keyword	Description
`SCFTYP=RHF`	Uses the Restricted Hartree–Fock method. Suitable for closed-shell molecules where all electrons are paired.
`RUNTYP=ENERGY`	Performs a single-point energy calculation without changing the molecular geometry.
`COORD=CART`	Indicates that the atomic coordinates are provided in Cartesian coordinates (x, y, z).
`UNITS=ANGS`	Specifies that all coordinates are given in Ångström (Å).

2. `$BASIS` Group¶

$BASIS GBASIS=STO NGAUSS=3 $END

The $BASIS group defines the basis set used to represent the molecular orbitals.

Keyword	Description
`GBASIS=STO`	Uses a Slater-Type Orbital (STO) basis represented by Gaussian functions.
`NGAUSS=3`	Uses three Gaussian primitives to approximate each Slater orbital, corresponding to the STO-3G basis set.

The STO-3G basis is a minimal basis set and is commonly used for educational examples and introductory calculations because it is computationally inexpensive.

3. `$SYSTEM` Group¶

$SYSTEM MWORDS=200 $END

This group controls the computational resources allocated to the calculation.

Keyword	Description
`MWORDS=200`	Allocates approximately 200 million words of memory for the calculation.

For small molecules like water, this amount of memory is more than sufficient.

4. `$GUESS` Group¶

$GUESS GUESS=HUCKEL $END

Before the SCF iterations begin, GAMESS requires an initial approximation to the molecular orbitals.

Keyword	Description
`GUESS=HUCKEL`	Generates the initial molecular orbitals using the Extended Hückel method.

A reasonable initial guess helps the SCF procedure converge more quickly.

5. `$DATA` Group¶

$DATA
Water Molecule
C1
O      8.0      0.0000000000    0.0000000000    0.1068300000
H      1.0      0.0000000000    0.7851780000   -0.4273190000
H      1.0      0.0000000000   -0.7851780000   -0.4273190000
$END

The $DATA group contains the molecular structure.

Line 1¶

Water Molecule

This is simply the title of the calculation and can be any descriptive text.

Line 2¶

C1

This specifies the point group symmetry of the molecule.

C1 indicates that no symmetry is being used. Although water belongs to the C₂v point group, using C1 simplifies the calculation and avoids complications related to symmetry.

Atomic Coordinates¶

Each remaining line defines one atom using the format

Atom   Atomic Number   X-coordinate   Y-coordinate   Z-coordinate

For example,

O      8.0      0.0000000000    0.0000000000    0.1068300000

represents an oxygen atom with

Atomic number = 8
Cartesian coordinates = (0.0000, 0.0000, 0.1068 Å)

Similarly,

H      1.0      0.0000000000    0.7851780000   -0.4273190000

defines one hydrogen atom with atomic number 1.

Summary¶

The complete input file tells GAMESS:

What method to use (RHF)
What type of calculation to perform (single-point energy)
Which basis set to use (STO-3G)
How to generate the initial molecular orbitals (Hückel guess)
The molecular geometry of the water molecule

Once this information is provided, GAMESS begins the Self-Consistent Field (SCF) procedure to compute the molecular orbitals and the total Hartree–Fock energy. ...