It is a well-known that as a trial wave function for the CASINO you can use HF or DFT wfn, the latter in many cases give a better WFN approximation.
I also tried to use orbital-optimized MP2 trial wave function but it's almost never better than DFT, at least it does not give significant improvements.
After that it's time to use multi-determinant trial wave function, this approach can improve DMC energy further, but the number of determinants that must be taken to obtain the exact energy is frightening.
I suppose that after taking into account the necessary number of determinants (in some cases, only one), when we get the correct topology of the nodal surface, we can not add new determinants, but use the backflow to improve the quality of the nodal surface. So I plan to use the backflow in my calculations next year.
In most cases, the generation of a multi-determinant wave function is performed using CI/CC methods, I can recall only a few simple systems such as Be, B, C atoms for which multi-determinant wave function can be generated from CASSCF or written by hands.
CI/CC methods requires the use of a high angular momentum basis (like cc-pVQZ, cc-pV5Z, cc-pV6Z) because of nature of methods. So applying the backflow after this also requires support as high orbital moment as possible.
Unfortunately, I noticed that backflow supports only up to f-orbitals.
I would like to know what plans for the new functionality of backflow next year.
I found use_gbackflow key word
Is still no effect of this?CASINO HELP SYSTEM
Keyword : use_gbackflow
Title : DEV: use gbackflow
Type : Boolean
Level : Expert
RESERVED KEYWORD FOR DEVELOPMENT, NO EFFECT.
How difficult is to support of g-orbitals in backflow, for me if I would start programming in FORTRAN?
How slower will the backflow calculation be with g-orbitals?
In the preliminary calculations, I found that it is sufficient to use only ETA and MU terms in backflow to improve MD WFN energy and I also making full electron calculations. How much does this simplify or complicate the task?