ISPIN controls the Spin-polarized and Spin-Unpolarized calculation in VASP code. But, for calculations (molecular), how to do a fixed spin-multiplicity calculation (as performed in Gaussian basis set based codes)?
Here is the way. Use NUPDOWN tag in INCAR.
NUPDOWN is the number of excess ELECTRONS, each with spin 1/2. So,
NUPDOWN = 0 => S=0, SINGLET
NUPDOWN = 1 => S=1/2, 2S+1 = 2 = DOUBLET
NUPDOWN = 2 +> S=1, 2S+1 = 3 = TRIPLET.
There is another discussion on doing spin-restricted and unrestricted singlet calculaiton.
An advice from experts is that you do a triplet calculation first.
"the way to go for the open shell singlet is to do a triplet calculation, save WAVECAR and CHGCAR, then put NUPDOWN = 0 into INCAR and restart reading the wavefunction. You'd have to check the DOS if you really ended up with an open shell singlet, or if it converges to degenerated alpha and beta anergies."
For more questions and answers, follow these links.
Singlet and triplet state calculations
Here is the way. Use NUPDOWN tag in INCAR.
NUPDOWN is the number of excess ELECTRONS, each with spin 1/2. So,
NUPDOWN = 0 => S=0, SINGLET
NUPDOWN = 1 => S=1/2, 2S+1 = 2 = DOUBLET
NUPDOWN = 2 +> S=1, 2S+1 = 3 = TRIPLET.
There is another discussion on doing spin-restricted and unrestricted singlet calculaiton.
An advice from experts is that you do a triplet calculation first.
"the way to go for the open shell singlet is to do a triplet calculation, save WAVECAR and CHGCAR, then put NUPDOWN = 0 into INCAR and restart reading the wavefunction. You'd have to check the DOS if you really ended up with an open shell singlet, or if it converges to degenerated alpha and beta anergies."
For more questions and answers, follow these links.
Singlet and triplet state calculations
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