  Inhomogeneous boson systems, such as the dilute gases of integral spin atoms
in low-temperature magnetic traps, are believed to be well described by the
Gross-Pitaevskii equation (GPE). GPE is a nonlinear Schroedinger equation which
describes the order parameter of such systems at the mean field level. In the
present work, we describe a Fortran 90 computer program developed by us, which
solves the GPE using a basis set expansion technique. In this technique, the
condensate wave function (order parameter) is expanded in terms of the
solutions of the simple-harmonic oscillator (SHO) characterizing the atomic
trap. Additionally, the same approach is also used to solve the problems in
which the trap is weakly anharmonic, and the anharmonic potential can be
expressed as a polynomial in the position operators x, y, and z. The resulting
eigenvalue problem is solved iteratively using either the self-consistent-field
(SCF) approach, or the imaginary time steepest-descent (SD) approach. Our
results for harmonic traps are also compared with those published by other
authors using different numerical approaches, and excellent agreement is
obtained. GPE is also solved for a few anharmonic potentials, and the influence
of anharmonicity on the condensate is discussed. Additionally, the notion of
Shannon entropy for the condensate wave function is defined and studied as a
function of the number of particles in the trap. It is demonstrated numerically
that the entropy increases with the particle number in a monotonic way.
