Q-Chem can be used a QM back-end for QM/MM calculations using Charmm package [649]. In this case, both software packages are required to perform the calculations, but all the code required for communication between the programs is incorporated in the released versions. Stand-alone QM/MM calculations are described in Section 11.3.

QM/MM jobs that utilize the Charmm interface are controlled using the following *$rem* keywords:

QM_MM

Turns on the Q-Chem/Charmm interface.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Do QM/MM calculation through the Q-Chem/Charmm interface.

FALSE

Turn this feature off.

RECOMMENDATION:

Use default unless running calculations with Charmm.

QMMM_PRINT

Controls the amount of output printed from a QM/MM job.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Limit molecule, point charge, and analysis printing.

FALSE

Normal printing.

RECOMMENDATION:

Use default unless running calculations with Charmm.

QMMM_CHARGES

Controls the printing of QM charges to file.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Writes a charges.dat file with the Mulliken charges from the QM region.

FALSE

No file written.

RECOMMENDATION:

Use default unless running calculations with Charmm where charges on the QM region need to be saved.

IGDEFIELD

Triggers the calculation of the electrostatic potential and/or the electric field at the positions of the MM charges.

TYPE:

INTEGER

DEFAULT:

UNDEFINED

OPTIONS:

O

Computes ESP.

1

Computes ESP and EFIELD.

2

Computes EFIELD.

RECOMMENDATION:

Must use this

$remwhen IGDESP is specified.

GEOM_PRINT

Controls the amount of geometric information printed at each step.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Prints out all geometric information; bond distances, angles, torsions.

FALSE

Normal printing of distance matrix.

RECOMMENDATION:

Use if you want to be able to quickly examine geometric parameters at the beginning and end of optimizations. Only prints in the beginning of single point energy calculations.

QMMM_FULL_HESSIAN

Trigger the evaluation of the full QM/MM Hessian.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Evaluates full Hessian.

FALSE

Hessian for QM-QM block only.

RECOMMENDATION:

None

LINK_ATOM_PROJECTION

Controls whether to perform a link-atom projection

TYPE:

LOGICAL

DEFAULT:

TRUE

OPTIONS:

TRUE

Performs the projection

FALSE

No projection

RECOMMENDATION:

Necessary in a full QM/MM Hessian evaluation on a system with link atoms

HESS_AND_GRAD

Enables the evaluation of both analytical gradient and Hessian in a single job

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

TRUE

Evaluates both gradient and Hessian.

FALSE

Evaluates Hessian only.

RECOMMENDATION:

Use only in a frequency (and thus Hessian) evaluation.

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

**Example 11.255** Do a basic QM/MM optimization of the water dimer. You need Charmm to do this but this is the Q-Chem file that is needed to test the QM/MM functionality. These are the bare necessities for a Q-Chem/Charmm QM/MM calculation.

$molecule 0 1 O -0.91126 1.09227 1.02007 H -1.75684 1.51867 1.28260 H -0.55929 1.74495 0.36940 $end $rem METHOD hf ! HF Exchange BASIS cc-pvdz ! Correlation Consistent Basis QM_MM true ! Turn on QM/MM calculation JOBTYPE force ! Need this for QM/MM optimizations $end $external_charges 1.20426 -0.64330 0.79922 -0.83400 1.01723 -1.36906 1.39217 0.41700 0.43830 -0.06644 0.91277 0.41700 $end

The Q-Chem/Charmm interface is unique in that:

The external point charges can be replaced with Gaussian-delocalized charges with a finite width [661]. This is an empirical way to include the delocalized character of the electron density of atoms in the MM region. This can be important for the electrostatic interaction of the QM region with nearby atoms in the MM region.

We allow the evaluation of the full QM/MM Hessian [530]. When link atoms are inserted to saturate the QM region, all Hessian elements associated with link atoms are automatically projected onto their QM and MM host atoms.

For systems with a large number of MM atoms, one can define blocks consisting of multiple MM atoms (

*i.e.*, mobile blocks) and efficiently evaluate the corresponding mobile-block Hessian (MBH) for normal mode analysis.