General variational many-body theory with complete self-consistency for trapped bosonic systems

TL;DR

This paper develops a comprehensive variational many-body theory for trapped bosonic systems, treating basis functions and coefficients as variational parameters, and emphasizes the importance of self-consistency for accurate physical predictions.

Contribution

It introduces the multi-configurational Hartree for bosons (MCHB(M)) method with self-consistent optimization of basis and coefficients, applicable to general two-body interactions.

Findings

Self-consistency significantly affects physical property predictions.

The method accurately describes ground and excited states.

Convergence is validated for two bosons in a double-well.

Abstract

In this work we develop a complete variational many-body theory for a system of $N$ trapped bosons interacting via a general two-body potential. In this theory both the many-body basis functions {\em and} the respective expansion coefficients are treated as variational parameters. The optimal variational parameters are obtained {\em self-consistently} by solving a coupled system of non-eigenvalue -- generally integro-differential -- equations to get the one-particle functions and by diagonalizing the secular matrix problem to find the expansion coefficients. We call this theory multi-configurational Hartree for bosons or MCHB(M), where M specifies explicitly the number of one-particle functions used to construct the configurations. General rules for evaluating the matrix elements of one- and two-particle operators are derived and applied to construct the secular Hamiltonian matrix. We discuss properties of the derived equations. It is demonstrated that for any practical computation where the configurational space is restricted, the description of trapped bosonic systems strongly depends on the choice of the many-body basis set used, i.e., self-consistency is of great relevance. As illustrative examples we consider bosonic systems trapped in one- and two-dimensional symmetric and asymmetric double-well potentials. We demonstrate that self-consistency has great impact on the predicted physical properties of the ground and excited states and show that the lack of self-consistency may lead to physically wrong predictions. The convergence of the general MCHB(M) scheme with a growing number M is validated in a specific case of two bosons trapped in a symmetric double-well.