FCI and GENCI initial core guess determinants for Davidson-Liu solver

[marink2]

Description

PR incorporates the feature to run a FCI (or GENCI) calculation(s) with core-excited determinants as the initial guess space for the Davidson-Liu solver. Core-excited determinants include determinant strings with single- and none-occupation on the first position.

User Notes

• Added default boolean parameter (bool core_guess) to StateInfo class.

• Added protected boolean member variable (core_guess_) to ActiveSpaceMethod class.

• ActiveSpaceSolver assigns values of DL_CORE_INITIAL_GUESS to StateInfo object, StateInfo maps this value to ActiveSpaceMethod, which passes value to GENCI or FCI solver.

• Feature is implemented for GENCI and FCI solvers only.

• Included two test cases, fci-core-1 and fci-core-2, for the implemented feature.

• Added default boolean parameter (bool core_guess) to find_initial_guess_det member function of Forte.
  This feature prints the value(s) of DL_CORE_INITIAL_GUESS in an appropriate place ("Initial Guess") of the output.dat file. To avoid cluttering the parameter list of this function consider creating an instance of ActiveSpaceMethod in sparce_ci_initial_guess.cc.

Checklist

• Added/updated tests of new features and included a reference output.ref file
• Documented source code
• Documented new features in the manual
• Ready to go!

[huangm29]

Let us change push_back to emplace_back here, though the speed does not matter at this place. I can barely find a place where emplace_back is worse than push_back

[huangm29]

I think we need a better name for this. Core_guess does not make sense to me. Making it longer and clearer what it means.

[huangm29]

Please ensure your "User Notes" and "Checklist" correspond to your changes. See other PR for examples.

[huangm29]

repr += "" + std::to_string(core_guess ? 1 : 0);

[huangm29]

Make some changes suggested by comments and these are good to go!