FORCE_MIXING

References: Bernstein2009, Bernstein2012

This section enables and defines parameters for force-mixing based QM/MM, which actually does two conventional QM/MM calculations, on a small and a large QM region, and combines the MM forces from one and QM forces from the other to create a complete set of forces. Energy is not conserved (although the QM/MM energy from the large QM region calculation is reported) so a proper thermostat (i.e. massive, and able to handle dissipation, such as Adaptive Langevin (AD_LANGEVIN)) must be used. For some propagation algorithms (NVT and REFTRAJ MD ensembles) algorithm is adaptive, including molecules hysteretically based on their instantaneous distance from the core region. Information on core/QM/buffer labels can be written in PDB file using MOTION&PRINT&FORCE_MIXING_LABELS. Will fail if calculation requires a meaningfull stress, or an energy that is consistent with the forces. For GEO_OPT this means only MOTION&GEO_OPT&TYPE CG, MOTION&GEO_OPT&CG&LINE_SEARCH&TYPE 2PNT, and MOTION&GEO_OPT&CG&LINE_SEARCH&2PNT&LINMIN_GRAD_ONLY T [Edit on GitHub]

Subsections

Keyword descriptions

SECTION_PARAMETERS

Type: logical
Default: F
Lone keyword: T

Description: Enables force-mixing

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ADAPTIVE_EXCLUDE_MOLECULES

Type: string[ ]
Usage: ADAPTIVE_EXCLUDE_MOLECULES molec1 molec2 …

Description: List of molecule names to exclude from adaptive regions (e.g. big things like proteins)

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EXTENDED_DELTA_CHARGE

Type: integer
Default: 0
Usage: EXTENDED_DELTA_CHARGE q

Description: Additional net charge in extended region relative to core (core charge is specified in DFT section, as usual for a convetional QM/MM calculation)

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MAX_N_QM

Type: integer
Default: 300
Usage: MAX_N_QM int

Description: Maximum number of QM atoms, for detection of runaway adaptive selection.

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MOMENTUM_CONSERVATION_REGION

Type: enum
Default: QM
Usage: MOMENTUM_CONSERVATION_REGION

Description: Region to apply correction force to for momentum conservation

Valid values:

CORE Apply to QM core region
QM Apply to full QM (dynamics) region
BUFFER Apply to QM+buffer regions

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MOMENTUM_CONSERVATION_TYPE

Type: enum
Default: EQUAL_A
Usage: MOMENTUM_CONSERVATION_TYPE

Description: How to apply force to get momentum conservation

Valid values:

NONE No momentum conservation
EQUAL_F Equal force on each atom
EQUAL_A Equal acceleration on each atom

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QM_EXTENDED_SEED_IS_ONLY_CORE_LIST

Type: logical
Default: F
Usage: QM_EXTENDED_SEED_IS_ONLY_CORE_LIST

Description: Makes the extended QM zone be defined hysterestically by distance from QM core list (i.e. atoms specified explicitly by user) instead of from full QM core region (specified by user + hysteretic selection + unbreakable bonds)

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QM_KIND_ELEMENT_MAPPING

Type: string[2]
Repeatable: yes
Usage: QM_KIND_ELEMENT_MAPPING {El} {QM_KIND}

Description: Mapping from elements to QM_KINDs for adaptively included atoms.

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R_BUF

Type: real[2]
Default: 5.00000000E-001 1.00000000E+000 [angstrom]
Usage: R_BUF

Description: Specify the inner and outer radii of buffer region. All atoms within this distance (hysteretically) of any QM atoms will be buffer atoms in the force-mixing calculation.

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R_CORE

Type: real[2]
Default: 0.00000000E+000 0.00000000E+000 [angstrom]
Usage: R_CORE

Description: Specify the inner and outer radii of core QM region. All molecules with any atoms within this distance (hysteretically) of any atoms specified as QM in enclosing QM/MM section will be core QM atoms in the force-mixing calculation.

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R_QM

Type: real[2]
Default: 5.00000000E-001 1.00000000E+000 [angstrom]
Usage: R_QM

Description: Specify the inner and outer radii of QM dynamics region. All molecules with atoms within this distance (hysteretically) of any atoms in core will follow QM dynamics in the force-mixing calculation.

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