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# 11.3.4 Additional Job Control Variables

(December 20, 2021)

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

FORCE_FIELD

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

CHARGE_CHARGE_REPULSION
The repulsive Coulomb interaction parameter for YinYang atoms.
TYPE:
INTEGER
DEFAULT:
550
OPTIONS:
$n$ Use Q = $n\times 10^{-3}$
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

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

GAUSS_BLUR_WIDTH
Delocalization width for external MM Gaussian charges in a Janus calculations.
TYPE:
INTEGER
DEFAULT:
NONE
OPTIONS:
$n$ Use a width of $n\times 10^{-4}$ 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.  247 Das D. et al.
J. Chem. Phys.
(2002), 117, pp. 10534.
.

MODEL_SYSTEM_CHARGE

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 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

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

MM_SUBTRACTIVE
Specifies whether a subtractive scheme is used in the $E_{\mathrm{Coul}}$, Eq. (11.51), 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.