Gaussian

Setting Up a Gaussian Calculation

program = gaussian
method  = B3LYP/6-31G*
nprocs  = 8
mem     = 16GB
charge  = 0
mult_a  = 1
mult_b  = 3

method Format

The method value is inserted directly into the Gaussian route section (# line). You can include any valid Gaussian route keywords:

method = n scf(maxcycle=500,xqc) uwb97xd/def2svpp scrf=(smd,solvent=acetonitrile)

mem Format

Specify memory using Gaussian's syntax — e.g. 4GB, 8GB, 4000MB.

Executable

By default, OpenMECP calls g16. Override with:

gau_comm = g09    # or full path: /opt/g16/g16

TD-DFT and Excited States

Use td_a / td_b to inject TD-DFT keywords into the route section for each state, and state_a / state_b to select which root to use for the gradient:

td_a   = TD(nstates=5,root=1)
td_b   = TD(nstates=5,root=2)
state_a = 1
state_b = 2

MP2

Enable MP2 gradients (instead of DFT) with:

mp2 = true

CASSCF and Multi-Reference Methods

Put CASSCF keywords in the *TAIL_A / *TAIL_B sections as extra route or additional input blocks:

*TAIL_A
casscf(8,8)/cc-pVDZ
*

*TAIL_B
casscf(8,8)/cc-pVDZ
*

Extra Input via *TAIL Sections

Anything in *TAIL_A / *TAIL_B is appended to the Gaussian route section keywords for the respective state. Common uses:

*TAIL_A
scf(maxcycle=500,xqc) stable=opt
*

ONIOM (QM/MM)

Multi-layer ONIOM calculations are supported. Mark atom layers in *GEOM using Gaussian's H / M / L layer notation:

*GEOM
C  0.0  0.0  0.0  H
H  1.0  0.0  0.0  L
H  0.0  1.0  0.0  L
*

isoniom = true
chargeandmultforoniom1 = 0 1 0 1   # state A: QM and MM charges/mults
chargeandmultforoniom2 = 0 3 0 1   # state B

Run Modes and Checkpoints

OpenMECP automatically manages Gaussian checkpoint files (.chk) to pass the wavefunction between steps. The mode keyword controls this behavior — see Run Modes for details.

The inter_read mode is essential for open-shell singlet (OSS) calculations: it copies the triplet wavefunction to the singlet and adds guess=(read,mix).

Output Files