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

• BUFFER_LINKS
• BUFFER_NON_ADAPTIVE
• PRINT
• QM_NON_ADAPTIVE
• RESTART_INFO

Keywords

• SECTION_PARAMETERS

• ADAPTIVE_EXCLUDE_MOLECULES

• EXTENDED_DELTA_CHARGE

• MAX_N_QM

• MOMENTUM_CONSERVATION_REGION

• MOMENTUM_CONSERVATION_TYPE

• QM_EXTENDED_SEED_IS_ONLY_CORE_LIST

• QM_KIND_ELEMENT_MAPPING

• R_BUF

• R_CORE

• R_QM

Keyword descriptions

SECTION_PARAMETERS: logical = F

Lone keyword: T

Enables force-mixing [Edit on GitHub]

ADAPTIVE_EXCLUDE_MOLECULES: string

Usage: ADAPTIVE_EXCLUDE_MOLECULES molec1 molec2 …

List of molecule names to exclude from adaptive regions (e.g. big things like proteins) [Edit on GitHub]

EXTENDED_DELTA_CHARGE: integer = 0

Usage: EXTENDED_DELTA_CHARGE q

Additional net charge in extended region relative to core (core charge is specified in DFT section, as usual for a convetional QM/MM calculation) [Edit on GitHub]

MAX_N_QM: integer = 300

Usage: MAX_N_QM int

Maximum number of QM atoms, for detection of runaway adaptive selection. [Edit on GitHub]

MOMENTUM_CONSERVATION_REGION: enum = QM

Usage: MOMENTUM_CONSERVATION_REGION

Valid values:

• CORE Apply to QM core region

• QM Apply to full QM (dynamics) region

• BUFFER Apply to QM+buffer regions

Region to apply correction force to for momentum conservation [Edit on GitHub]

MOMENTUM_CONSERVATION_TYPE: enum = EQUAL_A

Usage: MOMENTUM_CONSERVATION_TYPE

Valid values:

• NONE No momentum conservation

• EQUAL_F Equal force on each atom

• EQUAL_A Equal acceleration on each atom

How to apply force to get momentum conservation [Edit on GitHub]

QM_EXTENDED_SEED_IS_ONLY_CORE_LIST: logical = F

Usage: QM_EXTENDED_SEED_IS_ONLY_CORE_LIST

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) [Edit on GitHub]

QM_KIND_ELEMENT_MAPPING: string[2]

Keyword can be repeated.

Usage: QM_KIND_ELEMENT_MAPPING {El} {QM_KIND}

Mapping from elements to QM_KINDs for adaptively included atoms. [Edit on GitHub]

R_BUF: real[2] = 5.00000000E-001 1.00000000E+000 [angstrom]

Usage: R_BUF

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. [Edit on GitHub]

R_CORE: real[2] = 0.00000000E+000 0.00000000E+000 [angstrom]

Usage: R_CORE

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. [Edit on GitHub]

R_QM: real[2] = 5.00000000E-001 1.00000000E+000 [angstrom]

Usage: R_QM

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. [Edit on GitHub]