# 11.4 Q-CHEM/CHARMM Interface

Q-Chem can be used a QM back-end for QM/MM calculations using Charmm package.Woodcock:2007 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 use 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 the 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 the 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 the default unless running calculations with Charmm where charges on the QM region need to be saved. ESP_EFIELD Triggers the calculation of the electrostatic potential (ESP) and/or the electric field at the positions of the MM charges. TYPE: INTEGER DEFAULT: 0 OPTIONS: 0 Computes ESP only. 1 Computes ESP and electric field. 2 Computes electric field only. RECOMMENDATION: None. 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 SKIP_CHARGE_SELF_INTERACT Ignores the electrostatic interactions among external charges in a QM/MM calculation. TYPE: LOGICAL DEFAULT: FALSE OPTIONS: TRUE No electrostatic interactions among external charges. FALSE Computes the electrostatic interactions among external charges. RECOMMENDATION: None Example 11.22 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.Das:2002 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.Woodcock:2008 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.