Specifying the two-body potential $\hat{W}$:

The two-body interaction $\hat{W}$ is specified in the file Get_InterParticle_Potential.F in the directory source/ini_guess_pot/. This file contains a single Fortran subroutine with a case selection on the type of interparticle interaction and the evaluation of its action (Interaction_Type is the corresponding input variable). The standard types are a contact interaction potential (Interaction_Type=0,6) or short range Gaussian interaction (Interaction_Type=1,2,3,4,5). For these cases of a short-range interparticle interaction, the width of it can be adjusted with the input variable Interaction_Width in MCTDHX.inp. Generally, a non-contact interaction requires Interaction_Type > 0. Interaction_type=5 is a time-dependent non-contact interaction and Interaction_type=6 allows the simulation of contact interactions with a time-dependent interaction strength. The different evaluation types that Interaction_Type selects and the necessary properties of the interaction potentials are also specified in the html documentation and the example input in the directory Input_Examples. If a custom interaction potential is desired, this has to be implemented in the Get_InterParticle_Potential.F routine. The table in Appendix B shows the predefined interparticle interaction potentials as well as how to configure them with the input file's parameters.