\section*{Acknowledgements}

The research of P.A. was supported by Sistema Nacional de
Investigadores (M\'exico).

\section*{Addendum}

According to the reviewer: ``Another point to be observed is the
dependence of the Rayleigh-Ritz variational method on the choice
of the wave function. Despite not being mentioned by the authors
of this comment, the wave function used in the Rayleigh-Ritz
variational method is obtained from the asymptotic analysis made
by Vit\'{o}ria et al. If one uses another wave function that
differs from the wave function obtained from the asymptotic
analysis made by Vit\'{o}ria et al, therefore, the results will be
different. In addition, no mathematical proof has been shown in
this comment that clarifies the relation of the approximate
solutions to the biconfluent Heun equation.''

This comment is surprising. In order to apply the Ritz variational
method it is mandatory that the basis functions satisfy the
correct boundary conditions at $x=0$ and $x\rightarrow \infty $.
Therefore, we have chosen the simplest basis set that satisfy such
boundary conditions. The  set of Gaussian functions chosen here is
complete and, for this reason it should give the actual
eigenvalues of the problem at hand. This fact is clearly revealed
in the convergence of the approximate eigenvalues shown in Tables
\ref{tab:theta1} and \ref{tab:theta2}. The Ritz variational method
is well known and and has been widely used for the study of many
quantum-mechanical problems.


