Bad wavefunction with spin polarized SPO using LCAO

[shivupa]

After a discussion with Paul via email, I'm opening an issue on the following problem we've been having.

Relevant files are in the following gist.

The systems for which we have encountered problems are dipole-bound anions. These are systems in which an electron binds in the dipole field of the molecule. They have very extended charge distributions. We have been able to calculate electron binding energies for several of these systems using Casino, but attempts to do so with QMCPACK have failed. The most recent such system that we have attempted is CH3CN.

The neutral molecule presents no problems: the problems are with the anion. The calculations are all-electron with 1-body and 2-body Jastrow factors. Our most recent calculations used the 3.5.0 release of QMCPACK. These were all-electron calculations with cusp corrected orbitals using the standard cusp correction algorithm. The VMC without a Jastrow (but with cusp corrected orbitals) does agree with HF (-131.97 ± 0.19 for VMC and -131.939 for HF).

We first optimized the Jastrow parameters for the neutral using cutoffs optimized in Casino and then used these as starting parameters for the anion. However, the VMC optimization on the anion failed with variances ~ 10^14. The Jastrow factors used splines and were optimized using the OneShiftOnly optimizer.

Pure variance (both unreweighted and reweighted) minimization behaved similarly. In both optimization approaches the high variances begin from the first iteration.

I attached plots of the starting Jastrow factors. They were generated using a short optimization on the neutral. It seems like a reasonable starting guess.

We've tried another dipole-bound anion system, C3H2 and were similarly unsuccessful. In that system there is the possibility of collapsing to a valence bound anion which could cause complications so we switched to CH3CN. Similarly, a long time ago we tried a model water system which was also unstable.

The binding of the systems we are trying to treat is very diffuse compared to common anionic systems, but I am unsure if this would cause any complications for QMC.

Let me know if I can provide any other relevant information.

Thanks, Shiv