PCM Energy

Examples of Reference-SCF PCM/ddX Energy Calculations

The examples below show how to add a [pcm] section to OpenQP single-point energy inputs. The current PCM implementation is an energy-only reference-SCF path using the ddX backend. It is suitable for RHF/ROHF reference energies and for a reference-SCF MRSF-TDDFT PCM baseline. It is not a state-specific excited-state PCM correction and it does not provide PCM gradients or optimizations.

Closed-shell RHF reference-SCF ddPCM energy

[input]
system=
   8   0.000000000   0.000000000  -0.041061554
   1  -0.533194329   0.533194329  -0.614469223
   1   0.533194329  -0.533194329  -0.614469223
charge=0
runtype=energy
basis=6-31g*
method=hf

[guess]
type=huckel

[scf]
multiplicity=1
type=rhf

[pcm]
enabled=true
backend=ddx
mode=reference_scf
model=ddpcm
solvent=water
epsilon=78.3553
radii=uff

[input] Section

• system: Defines the molecular geometry. The example uses inline Cartesian coordinates for water. You can also store the same coordinates in an external .xyz file and set system=filename.xyz.

• charge: The total molecular charge. charge=0 means the molecule is neutral.

• runtype: Must be energy for the current PCM implementation. PCM gradients, Hessians, NACs, and optimizations require separate analytic derivative implementations and are not part of this first energy path.

• basis: The AO basis set. The example uses 6-31g*.

• method: hf selects the time-independent HF/DFT driver. With no functional keyword, this example is an RHF calculation. To run a DFT reference calculation instead, add a functional such as functional=bhhlyp while keeping method=hf.

[guess] Section

• type: The initial molecular-orbital guess. huckel uses the native Huckel-type guess as the SCF starting point.

[scf] Section

• multiplicity: The spin multiplicity of the reference. multiplicity=1 is appropriate for a closed-shell singlet reference such as water.

• type: The SCF reference type. rhf is used for closed-shell restricted Hartree-Fock/Kohn-Sham calculations.

[pcm] Section

• enabled: true turns on the PCM reaction-field contribution. Set enabled=false or omit the section for a gas-phase/vacuum calculation.

• backend: ddx selects the implemented OpenQP/ddX PCM backend.

• mode: reference_scf builds the continuum reaction field from the SCF reference density.

• model: ddpcm selects the ddPCM model used by the current examples.

• solvent: A solvent label. The numerical dielectric is controlled by epsilon, so set epsilon explicitly for the solvent you want.

• epsilon: The static dielectric constant. 78.3553 is the water dielectric used in the OpenQP PCM validation examples.

• radii: Cavity-radius keyword. Keep the default uff for standard inputs unless a development branch documents another option.

MRSF-TDDFT reference-SCF ddPCM energy baseline

[input]
system=
   8   0.000000000   0.000000000  -0.041061554
   1  -0.533194329   0.533194329  -0.614469223
   1   0.533194329  -0.533194329  -0.614469223
charge=0
runtype=energy
basis=6-31g*
functional=bhhlyp
method=tdhf

[guess]
type=huckel

[scf]
multiplicity=3
type=rohf

[tdhf]
type=mrsf
nstate=3

[pcm]
enabled=true
backend=ddx
mode=reference_scf
model=ddpcm
solvent=water
epsilon=78.3553
radii=uff

[input] Section

• runtype: energy requests a single-point energy calculation. This is the only runtype supported by the current PCM path.

• functional: bhhlyp is used here as a common MRSF-TDDFT functional.

• method: tdhf selects the TDHF/TDDFT response driver used for SF-TDDFT and MRSF-TDDFT.

[scf] Section

• multiplicity: 3 creates the high-spin triplet reference normally used for MRSF-TDDFT.

• type: rohf selects a restricted open-shell reference. The current PCM energy path supports RHF and ROHF references, so this is the MRSF-compatible reference choice.

[tdhf] Section

• type: mrsf selects mixed-reference spin-flip TDDFT.

• nstate: Number of MRSF response states to compute.

[pcm] Section

• The PCM keywords have the same meaning as in the RHF example above.
• In this MRSF input, mode=reference_scf means the solvent reaction field is generated from the high-spin ROHF reference density. The response calculation then uses this reference-SCF PCM baseline. This is not a separate state-specific PCM treatment for each MRSF state.

What to check in the output

For PCM-enabled runs, the log reports PCM diagnostic lines such as the PCM energy contribution. The total SCF energy includes the PCM reaction-field contribution when enabled=true. If OpenQP was built without ddX support, the run stops with a clear error instead of silently producing a vacuum result.

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