12.3 Stand-Alone QM/MM Calculations

12.3.3 Additional Job Control Variables

A QM/MM job is requested by setting the $rem variables QM_MM_INTERFACE and FORCE_FIELD. Also required are a $qm_atoms input section and appropriate modifications to the $molecule section, as described above. Additional job control variables are detailed here.

QM_MM_INTERFACE
       Enables internal QM/MM calculations.
TYPE:
       STRING
DEFAULT:
       NONE
OPTIONS:
       MM Molecular mechanics calculation (i.e., no QM region) ONIOM QM/MM calculation using two-layer mechanical embedding JANUS QM/MM calculation using electronic embedding
RECOMMENDATION:
       The ONIOM model and Janus models are described above. Choosing MM leads to no electronic structure calculation. However, when using MM, one still needs to define the $rem variables BASIS and EXCHANGE in order for Q-Chem to proceed smoothly.

FORCE_FIELD
       Specifies the force field for MM energies in QM/MM calculations.
TYPE:
       STRING
DEFAULT:
       NONE
OPTIONS:
       AMBER99 AMBER99 force field CHARMM27 CHARMM27 force field OPLSAA OPLSAA force field
RECOMMENDATION:
       None.

CHARGE_CHARGE_REPULSION
       The repulsive Coulomb interaction parameter for YinYang atoms.
TYPE:
       INTEGER
DEFAULT:
       550
OPTIONS:
       n Use Q = n×10-3
RECOMMENDATION:
       The repulsive Coulomb potential maintains bond lengths involving YinYang atoms with the potential V(r)=Q/r. The default is parameterized for carbon atoms.

GAUSSIAN_BLUR
       Enables the use of Gaussian-delocalized external charges in a QM/MM calculation.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Delocalizes external charges with Gaussian functions. FALSE Point charges
RECOMMENDATION:
       None

GAUSS_BLUR_WIDTH
       Delocalization width for external MM Gaussian charges in a Janus calculations.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n Use a width of n×10-4 Å.
RECOMMENDATION:
       Blur all MM external charges in a QM/MM calculation with the specified width. Gaussian blurring is currently incompatible with PCM calculations. Values of 1.0–2.0 Å are recommended in Ref. 208.

MODEL_SYSTEM_CHARGE
       Specifies the QM subsystem charge if different from the $molecule section.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n The charge of the QM subsystem.
RECOMMENDATION:
       This option only needs to be used if the QM subsystem (model system) has a charge that is different from the total system charge.

MODEL_SYSTEM_MULT
       Specifies the QM subsystem multiplicity if different from the $molecule section.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n The multiplicity of the QM subsystem.
RECOMMENDATION:
       This option only needs to be used if the QM subsystem (model system) has a multiplicity that is different from the total system multiplicity. ONIOM calculations must be closed shell.

USER_CONNECT
       Enables explicitly defined bonds.
TYPE:
       STRING
DEFAULT:
       FALSE
OPTIONS:
       TRUE Bond connectivity is read from the $molecule section FALSE Bond connectivity is determined by atom proximity
RECOMMENDATION:
       Set to TRUE if bond connectivity is known, in which case this connectivity must be specified in the $molecule section. This greatly accelerates MM calculations.

MM_SUBTRACTIVE
       Specifies whether a subtractive scheme is used in the ECoul, Eq. (12.38), portion of the calculation.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       FALSE Only pairs that are not 1-2, 1-3, or 1-4 pairs are used. TRUE All pairs are calculated, and then the pairs that are double counted (1-2, 1-3, and 1-4) are subtracted out.
RECOMMENDATION:
       When running QM/MM or MM calculations there is not recommendation. When running a QM/MM-Ewald calculation the value must be set to TRUE.