SWEXPHC: Variational bound state solution for the three-body nonrelativistic Schrödinger equation

A Fortran package SWEXPHC is presented, designed to calculate nonrelativistic energies for the bound-state three-body problem with Coulomb interaction. The implementation is based on MPFUN2020 package written by D.H. Bailey, which allows calculations with arbitrary precision, where the number of working digits can be adjusted by the user. The approximate wave function is chosen in the form of a variational exponential expansion, which has proven itself over many years as an effective method for obtaining highly accurate solutions for various three-particle systems such as the helium atom and/or the molecular hydrogen ion.