Cutoff lengths in the Jastrow factor

General discussion of the Cambridge quantum Monte Carlo code CASINO; how to install and setup; how to use it; what it does; applications.
Neil Drummond
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Re: Cutoff lengths in the Jastrow factor

Post by Neil Drummond »

Dear Varelse,

The cutoff length behaviour you describe seems a bit unusual; nevertheless every system is unique and, if the energy expectation value is going down, one cannot argue with the variational principle. If you are looking at similar systems then I would certainly make use of what you have learned in this case when choosing initial cutoff lengths.

The DMC energy is in principle independent of the Jastrow factor, because the Jastrow factor does not affect the nodal surface. If you have a poor wave function then the time-step bias, population-control bias, etc. will be larger. Nevertheless, for an all-electron system, if you extrapolate to zero time step, you should get exactly the same DMC energy without a Jastrow factor as you do with a Jastrow factor (optimal or otherwise). If you are performing pseudopotential calculations then there is an additional pseudopotential locality approximation that is second order in the error in the wave function, so the DMC energy has a slight dependence on the Jastrow factor.

So, while a good Jastrow factor is undoubtedly helpful (and bad trial wave functions are responsible for most problems encountered in QMC calculations), it should nevertheless be the case that optimisation is a small fraction of your total CPU time - if you find you are using more core hours on optimisation than on the subsequent DMC then you are probably overdoing the optimisation.

If you have DMC results you can judge the quality of your Jastrow factor by evaluating (E_HFVMC - E_VMC)/(E_HFVMC - E_DMC), where E_HFVMC is the VMC energy without a Jastrow factor, E_VMC is the VMC energy with a Jastrow factor and E_DMC is the DMC energy. For single atoms and electron gases this should be about 95% or more; for molecules and crystals with pseudopotentials this should be more than, say, 85-90%; for all-electron systems it should be more than, say, 80-85%.

Best wishes,

Neil.
Vladimir_Konjkov
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Joined: Wed Apr 15, 2015 3:14 pm

Re: Cutoff lengths in the Jastrow factor

Post by Vladimir_Konjkov »

Hi, all.

I find that main disadvantage of Jastrow optimisation in finite system (molecule) that it's poorly describe inhomogeneity of electron density.
In Casino implementation, the only way to take the inhomogeneity into account is to introduce sets of different Jastrow parameters and cutoff the areas around points in space where these sets will act.
Practically the points in space are atoms in the molecule, but why not otherwise?
Is it possible in CASINO to add a set of dummy atoms (without orbitals, charges and cusp condition) which only serves as the centers of CHI TERM (and maybe F TERM)?
How it can improve the quality of Jastrow optimisation?

Vladimir.

P.S.
It's possible create set of dummy atoms.
No improvements have happened.

P.P.S
anyone interested can download a sample with dummy atoms
Attachments
dummy.tgz
dummy centres example
(881.16 KiB) Downloaded 661 times
Last edited by Vladimir_Konjkov on Tue Aug 04, 2015 3:15 am, edited 1 time in total.
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Neil Drummond
Posts: 113
Joined: Fri May 31, 2013 10:42 am
Location: Lancaster
Contact:

Re: Cutoff lengths in the Jastrow factor

Post by Neil Drummond »

Dear Vladimir,

Your postscriptum answers your question...

Best wishes,

Neil.
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