Q-Chem can be used a QM back-end for QM/MM calculations using Charmm package.404 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 12.3.
QM/MM jobs that use the Charmm interface are controlled using the following $rem keywords:
$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.208 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.1003 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.