ORCA

Setting Up an ORCA Calculation

program = orca
method  = B3LYP def2-SVP
nprocs  = 8
mem     = 8000
charge  = 0
mult_a  = 1
mult_b  = 3

`method` Format

The method value is inserted after the ! on the ORCA simple input line. Include all keywords that would normally follow ! in your ORCA input:

method = B3LYP def2-SVP CPCM(2-octanone) VeryTightSCF

`mem` Format

Specify per-core memory in MB — this maps to %maxcore in the ORCA input:

mem = 8000    # 8000 MB per core

Executable

By default, OpenMECP calls orca. Override with:

orca_comm = /opt/orca5/orca    # full path recommended for ORCA

TD-DFT and Excited States

Use the *TAIL_A / *TAIL_B sections to inject %tddft blocks:

*TAIL_A
%tddft
  nroots 5
  iroot  1
end
*

*TAIL_B
%tddft
  nroots 5
  iroot  2
end
*

Combined with:

state_a = 1
state_b = 2

CASSCF

Put CASSCF blocks in the tail sections:

*TAIL_A
%casscf
  nel    8
  norb   8
  nroots 3
  iroot  0
end
*

Extra Input via *TAIL Sections

Anything in *TAIL_A / *TAIL_B is appended as an additional input block for the respective state. Common uses:

*TAIL_A
%scf
  MaxIter 500
  Convergence VeryTight
end
*

Output Files

File	Content
`running_dir/{basename}_N_state_A.inp`	ORCA input for state A at step N
`running_dir/{basename}_N_state_A.out`	ORCA output for state A at step N
`running_dir/{basename}_N_state_A.engrad`	Gradient file for state A at step N
`state_A.gbw`	Wavefunction for state A (updated each step)
`state_B.gbw`	Wavefunction for state B (updated each step)
`{basename}_mecp.xyz`	Final MECP geometry

Run Modes

ORCA reads wavefunction from .gbw files. The read and noread run modes control whether OpenMECP passes a MORead keyword to ORCA. See Run Modes for details.

Notes

Ensure the orca executable is on your PATH or specify the full path via orca_comm.
ORCA requires an exclusive MPI launch; do not wrap it in mpirun in your job script — ORCA handles parallel execution internally.
The .engrad file is written only when ORCA is asked for gradients (the default for MECP runs).

OpenMECP Documentation