Bunlsi—a fortran program for solution of systems of nonlinear algebraic equations. Improved approaches to critical topics in the solution of systems of nonlinear equations are presented, including a new objective function, defined to ensure more reliable convergence tests; an efficient procedure for updating the Jacobian matrix; a bidimensional search, proposed to replace Newton’s iteration when it fails; and finally, a procedure avoiding difficulties due to ill-conditioned Jacobians.