PBEsol vs PBEsolxc

job314

Hi all, it's more of an functional question. I noticed that only limited subset of functionals is available for Raman intensity calculations. I have a molecular crystal and trying all of it, started with range separated, then hybrid... Now I found that PBE can be combined with D3 and trying that one. So my question is - what is the difference between PBE and PBEXC or PBEsol and PBEsolXC?

Turn out I can simply specify

DFT
PBE-D3
END

but not

DFT
PBEsol
END

the latter aborts and I can only make it work if I specify otherwise it returns an error

DFT
PBEsolxc
END

(EXCHANGE)[CORRELATION] FUNCTIONAL:(PBEsol GGA)[PBEsol]

The former, however, still returns

(EXCHANGE)[CORRELATION] FUNCTIONAL:(PERDEW-BURKE-ERNZERHOF)[PERDEW-BURKE-ERNZERHOF]

I guess my question is why PBE can specified as a single keyword but not PBEsol which needs to be PBEsolXC. What is the significance of xc addition here?

thank you

aerba

Hi,

In CRYSTAL, the most general syntax to specify exchange-correlation (xc) functionals is, within the DFT input block through the EXCHANGE and CORRELAT keywords as:

DFT
EXCHANGE
label of exchange functional (e.g. VBH, BECKE, PBE, ...)
CORRELAT
label of correlation functional (e.g. VWN, LYP, PBE, ...)
ENDDFT

For certain standard combinations of exchange and correlation functionals, we have implemented single keywords. For instance:

DFT
BLYP
ENDDFT

is equivalent to

DFT
EXCHANGE
BECKE
CORRELAT
LYP
ENDDFT

Another example (here the "XC" letters are appended at the end of the name to reflect that the functional is used for both the exchange and correlation part):

DFT
PBESOLXC
ENDDFT

is equivalent to

DFT
EXCHANGE
PBESOL
CORRELAT
PBESOL
ENDDFT

For PBE, there are three equivalent ways to define it in CRYSTAL:

DFT
PBEXC
ENDDFT

or

DFT
PBE
ENDDFT

or

DFT
EXCHANGE
PBE
CORRELAT
PBE
ENDDFT

There isn't a general rule, I'm afraid. This syntax aspects are discussed at page 134 of CRYSTAL23 User's Manual.

Hope this clarifies things a little,