Analytical Force for ECDA

In our recent work [J. Chem. Phys. 151, 134101 (2019)], we reformulated ECDA to be a fully variational method and derived its analytical forces. This makes ECDA efficient for structure-related calculations, such as geometry relaxations and phonon calculations.

Analytical forces are demonstrated by calculating the x-component force on Si2 in the Si2H8 molecule, for different Si-Si bond lengths. Benchmark is from using the finite difference method by displacing Si2 in the x direction.