Description

Currently, COSX in Psi4 uses a two-grid scheme to complete the SCF - the SCF is first converged on a smaller grid, and then a single iteration is performed on a larger grid, a scheme proposed by Neese. This scheme works quite well for energies; but since the SCF isn't converged on the larger grid, it may not be as ideal if wavefunction properties (e.g., gradients) are desired.

This PR fixes that by adding a new keyword, COSX_MAXITER_FINAL, an integer that determines the maximum number of SCF iterations to run on the final COSX grid. By default, this keyword is set to 1, mimicking the current behavior of the code. A more detailed description of behavior can be broken down as follows:

• If COSX_MAXITER_FINAL < 0; the SCF will attempt to fully converge on the final COSX grid (within the constraints provided by MAXITER, of course).
• If COSX_MAXITER_FINAL == 0; the SCF will not run on the final grid at all, effectively turning the COSX algorithm into a single-grid implementation.
• If COSX_MAXITER_FINAL > 0; the SCF will run on the final grid for a maximum number of iterations specified by the keyword.
  • If the SCF converges on the final grid before the iteration count specified by COSX_MAXITER_FINAL, the SCF will exit upon convergence.
  • If the iteration count specified by COSX_MAXITER_FINAL is reached before the SCF is converged on the final grid, the SCF will exit, treated as a success.
  • If the MAXITER SCF iteration limit is hit before the COSX_MAXITER_FINAL limit is reached, the calculation will fail.

Also of note, this PR changes CompositeJK so that early_screening is disabled for LinK-based composite methods. Practically, this means that LinK-based methods do not run the extra post-early-screening SCF iteration that is present in COSX-based methods.

User API & Changelog headlines

• Adds a new keyword, COSX_MAXITER_FINAL, for controlling the maximum number of SCF iterations to run on the final COSX grid.

Dev notes & details

• Adds a new keyword, COSX_MAXITER_FINAL, for controlling the maximum number of SCF iterations to run on the final COSX grid. The keyword can be used to run a preset number of SCF iterations or fully converge the SCF on the final grid, or to skip the final grid entirely.
• Changes the early screening feature so that it is only turned on for COSX-based composite methods.

Questions

• N/A

Checklist

• Tests added for any new features
• All or relevant fraction of full tests run

Status

• Ready for review
• Ready for merge