Q-Chem 4.3 User’s Manual

11.4 Q-CHEM/CHARMM Interface

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 $rem when 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: