From: Gerard Rowe (GerardR_at_usca.edu)
Date: Mon Feb 25 2019 - 08:52:58 CST
When running BS calculations in Orca, there are two ways to go:
1. creating a BS guess for each calculation or
2. reading a previously converged BS wavefunction.
Proceeding with the default settings in NAMD QM/MM, you'll always follow option #1. In that kind of calculation, you specify the high-spin multiplicity in the charge/multiplicity line, and input the BS settings in the %SCF block. This is somewhat wasteful, though, because every step of your optimization has to find the BS solution.
On the other hand, if you perform a single-point BS calculation and use its resulting GBW file as the input for the next calculation, you would set the spin multiplicity to whatever your BS solution was set to (if that state happens to be a singlet, make sure you specify UKS in the input line).
The hard part is tricking NAMD into reading the output of a single point calculation during the QM/MM run. The procedure I figured out is awkward, but it works. First, run a QM/MM calculation as your normally would, and kill the job once you see that Orca has been launched. Then, go into the folder containing the orca input file. Copy this input file along with the pointcharge file into another folder. Modify the Orca input to perform a BS calculation. Once this single-point calculation is complete, copy the gbw file into the scratch folder that NAMD set up. NAMD doesn't clean up these folders between runs or between iterations; it only overwrites files as necessary. Make sure the gbw file name matches the QM input file name (it's something like tmpout, if I recall correctly).
>From here, you will edit your namd input to specify the BS multiplicity, and remove any broken symmetry/flipspin commands from the %SCF block. You're reading in a converged wavefunction, so it will start with the BS state. During the QM/MM run, you should dump the Orca output to another text file so you can keep an eye on the Mulliken spin populations to ensure you are still working on the BS surface.
From: owner-namd-l_at_ks.uiuc.edu <owner-namd-l_at_ks.uiuc.edu> on behalf of Francesco Pietra <chiendarret_at_gmail.com>
Sent: Saturday, February 23, 2019 3:34 PM
Subject: namd-l: Fwd: QM-MM NAMD-ORCA broken symmetry
---------- Forwarded message ---------
From: Francesco Pietra <chiendarret_at_gmail.com<mailto:chiendarret_at_gmail.com>>
Date: Sat, Feb 23, 2019 at 5:38 PM
Subject: QM-MM NAMD-ORCA broken symmetry
To: NAMD <namd-l_at_ks.uiuc.edu<mailto:namd-l_at_ks.uiuc.edu>>
Sorry, orca requests nr electrons
Setting broken symmetry DFT with ORCA requires a block with multiplicity for both parts. Setting that, and commenting out the line specifying multiplicity in NAMD conf file, leads to crash. NAMD wants to see the multiplicity.
Is that any reason not to do like with MOPAC, i.e., setting the multiplicity directly in the block to the QM program?
As far as I can see in the lab, my system is bs, which can not be treated with semiempirical.
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