Old (removed) functionality

Unused and not useful functionality is occasionally removed from HANDE, in order to remove the maintenance burden for code that really has no benefit. In general, keeping failed experiments in the codebase is not helpful to developers (more work) and users (not obvious if an option should or should not be used). When it transpires that something falls into the category, we may hence remove it and detail it below. If you are interested in resurrecting this functionality, please dig through the git history and/or speak to a developer.

folded-spectrum FCIQMC

The folded-spectrum approach allows, in principle, access to excited states in FCIQMC via using the Hamiltonian \((H-\epsilon)^2\), where \(\epsilon\) is an energy offset. It emerged in practice to be very painful/impossible to converge to excited states for systems beyond the reach of conventional FCI.

defining an initiator determinant via a complete active space

Originally the initiator space was defined by a population threshold and a complete active space (CAS). It turns out that it is simpler to allow the initiator space to emerge naturally just through the population threshold (as used in later studies), whereas defining a CAS that is small but effective is not easy in large systems. Furthermore, using just a population threshold makes the initiator approximation easier to extend to other algorithms (i.e. CCMC and DMQMC).