Built-in Water QM/MM

TeraChem ships with a small built-in MM engine that can apply common water force fields to the MM region. This mode is the simplest QM/MM option in TeraChem — it does not depend on Amber or any external topology file — but it is restricted to water in the MM region. For arbitrary biomolecular topologies, use the OpenMM/Amber driver instead.

Input layout

A built-in water QM/MM job has the QM atoms in the usual coordinates file and the MM water atoms in a separate file pointed to by qmmm:

basis        6-31g
method       b3lyp
coordinates  qmwater.xyz
qmmm         mmwater.xyz
run          minimize
end

The MM water file must list atoms grouped by molecule, in the order O, H, H for each water. Hence the file length is always a multiple of three plus the usual two-line XYZ header:

mmwater.xyz

3

 O   6.450776    0.962914  -3.536427
 H   6.338156    1.514355  -2.771897
 H   6.519847    1.524502  -4.303300

The QM atoms in coordinates are independent — they need not be water. A common use case is a QM solute (or a single QM water) embedded in an MM water shell, as in this minimal QM-water/MM-water gradient test:

qmmm_water_pair.in

coordinates  water_qm.xyz
qmmm         water_mm.xyz
method       hf
basis        3-21g
charge       0
water        spc/e
threall      1e-14
convthre     1e-7
run          gradient
end

Supported water models

The MM water force field is selected with the water keyword. The supported values are:

Value	Model	Rigid by default?
`tip3p`	TIP3P	yes (rigid only)
`spc`	SPC	yes (rigid only)
`spc/e`	SPC/E	yes (rigid only)
`spc/fw`	SPC/Fw (flexible)	no — controlled by `rigidbonds`
`tip4p`	TIP4P	yes (rigid only)
`swm4`	SWM4	yes (rigid only)
`swm4-dp`	SWM4-DP (Drude polarizable, original)	yes (rigid only)
`swm4-ndp`	SWM4-NDP (Drude polarizable, negative-Drude)	yes (rigid only)

If water is omitted, TIP3P is used. Only the spc/fw model honors the rigidbonds keyword (TeraChem's other built-in models are inherently rigid).

Summary of keywords

Keyword	Type	Default	Description
`qmmm`	filename	not set	File containing MM water coordinates (O, H, H per molecule). Setting this keyword activates the built-in water QM/MM mode.
`water`	string	`tip3p`	Water model: `tip3p`, `spc`, `spc/e`, `spc/fw`, `tip4p`, `swm4`, `swm4-dp`, `swm4-ndp`
`rigidbonds`	bool	true	Constrain O-H bonds. Only effective for `spc/fw`; the other models are rigid regardless.