input parameter choice

[Moser Mario]

Hello,

i managed to install gromacs/cp2k and was able to simulate some systems, im still quite new to it and i had a few question about some parameters you can chose in input file.

I simulated a system with  Zn2+ with an histidine and in the &OT section i had only MINIMIZER, STEPSIZE and PRECONDITIONNER FULL_ALL.

(in gromacs cp2k the input file is generated automaticly with the mdp file but you can modify the cp2k.inp file and provided any inp file you want)

Then i wanted to simulate the exact same system but with Cu2+ instead of Zn2+, this time in the &OT section i had UKS that appeared, i looked in manual and UKS is using alpha and beta orbitals for spin polarized calculations and ROKS is using a restricted Kohn-sham calculation.

I got advised to put ROKS instead of UKS, is this because Cu2+ has an unpaired electron in orbital d (3d9) so we need to change multiplicity to 2 and restrict the DFT calculation ? (compared to Zn2+ who has no unpaired electron so multiplicity is set to 1)

Is there a more detailed version of the online manual that i missed or a book that could explain the parameters ?

Best regards,

Mario

[Krack Matthias (PSI)]

Hello Mario

 

UKS (alias LSD) is the first choice for systems with an odd number of electrons or if an overall multiplicity greater than one is required. ROKS is special and not all feature available with UKS are also implemented with ROKS. ROKS employs only one set of spatial orbitals which can be either doubly or singly occupied (or empty, of course) whereas UKS works with two sets of singly occupied spin orbitals, alpha and beta, i.e. ROKS implies compared to UKS a constraint concerning the spatial wavefunctions. You can find further details in any quantum chemistry textbook like the Szabo and Ostlund.

 

HTH

 

Matthias

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[Moser Mario]

Hello Dr.Krack,

thank you for you quick answer, for the book , is this specific topic treated in the chapter 2 : 2.5 Spin-Adapted Configurations ?

best,

Mario.

[Krack Matthias (PSI)]

Yes.

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