OPLSAA andheroget t

Dear Qchem Experts,
Can I use the OPLSAA force field AND standard quantum chemical method (DFT) in a combined optimization? i.e. to have a molecule (DFT) surrounded by explicit solvent molecules (OPLAA force field)? Simple in doesn’t seem to work.

Here is a very simple example of a water dimer, one molecule QM (PBE0 functional) and the other MM (OPLS force field). Works with either JANUS or ONIOM QM/MM interface. This is sample job QMMM_oniom_opt.in.

Example of an ONIOM optimization on a water dimer.

exchange pbe0
basis 6-31G*
qm_mm_interface oniom ! could be janus instead
force_field oplsaa
user_connect true
jobtype opt
molden_format true

4 5 6

0 1
O -0.790909 1.149780 0.907453 186 2 3 0 0
H -1.628044 1.245320 1.376372 187 1 0 0 0
H -0.669346 1.913705 0.331002 187 1 0 0 0
O 1.178001 -0.686227 0.841306 186 5 6 0 0
H 0.870001 -1.337091 1.468215 187 4 0 0 0
H 0.472696 -0.008397 0.851892 187 4 0 0 0

If you have a large number of MM water molecules, then you may want to use the L-BFGS optimizer, which is turned on by adding something like


(see sample job lbfgs_qmmm.in). By default, Q-Chem’s geometry optimizers use delocalized internal coordinates and a quasi-Newton optimization algorithm, which is a good choice for efficient location of the minimum but requires diagonalization of a Hessian-like matrix. For QM-only jobs where system size is severely limited by QM cost, that diagonalization step seldom becomes a bottleneck but that’s not true for QM/MM, where total (QM + MM) system size can be quite large. In such cases, the L-BFGS algorithm offers a limited-memory alternative, albeit at the expense of a generally much larger number of optimization steps.

How did you get the atom type numbers?

They are in the force field files that either exist, or can be placed, in QCAUX/force_field

how can i access them?

They are in the QCAUX directory in your local installation of Q-Chem, same place where basis set information is stored (different sub-directory).

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