# 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 <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 <logical>

  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 <real> <real>

  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 <real> <real>

  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 <real> <real>

  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]