From: Gerard Rowe (GerardR_at_usca.edu)
Date: Mon Nov 05 2018 - 07:32:06 CST
I don't think Orca has somehting like that. Gaussian has a feature they call ONIOM that can achieve what you describe. However, for bonded atoms that are the focus of the calculation, it's not a good idea to use different levels of theory because the boundary atoms won't behave correctly. Orca does have some computational shortcuts you can use to speed up the calculation, though. If you use a pure DFT functional like BP86 or M06L, you can take advantage of the RI approximation for a massive speedup with little loss in accuracy. You also want to use as small of a basis set as possible, which is probably going to be "sv" or "def2-svp".
See this page for a quick tutorial.
The Orca manual is much more extensive and explains the keywords better.
From: owner-namd-l_at_ks.uiuc.edu <owner-namd-l_at_ks.uiuc.edu> on behalf of McGuire, Kelly <mcg05004_at_byui.edu>
Sent: Sunday, November 4, 2018 1:59:51 PM
Subject: namd-l: QM/MM Question
A bit of an ignorant question, but does anyone know if it's possible with the NAMD QM/MM to use different theories with different atoms in the QM region? I ask because I have a divalent copper ion, 12 water molecules, and four histidine sidechains in my QM region. MP2 or HF is taking a very long time to complete just one step. ORCA doesn't have PM7. I am using MOPAC's PM7, but I'm not sure when we'll be able to open mopac output files in VMD to view the orbitals. So, with ORCA QM/MM, can I maybe use PM3 for the waters and histidines, and assign copper with MP2 or HF in the same run? Or would that be bad to use different levels of theory in the same region? Thanks!
Kelly L. McGuire
Department of Physiology and Developmental Biology
Brigham Young University
Provo, UT 84602
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