/*! \page BCS_wfDoc BCS

Keyword: BCS

\section description Description

Projections of the BCS wave function on a subspace of fixed number of
particles. It has a form of a determinant of a pair orbital 
\f$ \phi({\bf r}_i,{\bf r}_j) \f$,
\f[
\Psi({\bf r}_{1\uparrow},\ldots,{\bf r}_{N\uparrow},
     {\bf r}_{1\downarrow},\ldots,{\bf r}_{N\downarrow})
=\det\left[ \begin{array}{ccc}
\phi({\bf r}_1,{\bf r}_1)             & \cdots & \phi({\bf r}_{N\uparrow},{\bf r}_1)\\
\vdots                                & \ddots & \vdots \\
\phi({\bf r}_1,{\bf r}_{N\downarrow}) & \cdots & \phi({\bf r}_{N\uparrow},{\bf r}_{N\downarrow})
\end{array}\right] .
\f]
Only spin-unpolarized situations, \f$ N\uparrow = N\downarrow \f$,
can be accomodated. Moreover, BCS wave function is implemented <i>only
for homogeneous systems</i>, where the pair orbital depends only on
a single vector, i.e., the distance of the two particles,
\f$ \phi({\bf r}_i,{\bf r}_j) = \phi({\bf r}_i-{\bf r}_j) \f$. For
description of more general cases see \ref Pfaff_wfDoc wave function.

\section options Options

\subsection reqopt Required 

<table>
<tr> <th> Option <th> Type <th> Description
<tr> <td> PAIR_ORBITAL <td> Section <td> A section containing
definition of one or several \ref Jastrow_groupDoc s that construct the
pair orbital \f$ \phi({\bf r}_i-{\bf r}_j) \f$ in the same way as the \ref
Jastrow2_wfDoc factor is constructed. Only two-body terms are taken
into account, others are ignored.
</table>

\subsection optopt Optional

None.

\section examples Examples

A very simple case where the pair orbital is a linear combination of
just two \ref Poly_pade_functionDoc s.

<pre>
BCS
PAIR_ORBITAL { 
  JASTROW2
  GROUP { 
    TWOBODY { 
      COEFFICIENTS { 1.0 1.0  } 
    } 
    EEBASIS { 
      EE
      POLYPADE
      RCUT 12.1877
      BETA0 -0.8
      NFUNC 2
    }
  }
}
</pre>

A more involved example where the pair orbital is constructed as a
linear combination of two shells of plane waves. \ref
Basis_GroupsDoc construct is utilized to enforce isotropic orbital and
to reduce the number of parameters to just two.

<pre>
BCS
PAIR_ORBITAL { 
  JASTROW2
  GROUP { 
    TWOBODY { 
      COEFFICIENTS { 1.0 1.0 } 
    } 
    EEBASIS { 
      EE
      BASIS_GROUPS
      BASIS_GROUP { 
        EE
        PLANEWAVE
        GVECTOR { 
          0.0       0.0      0.0
        }
      }
      BASIS_GROUP { 
        EE
        PLANEWAVE
        GVECTOR { 
          0.0       0.0       0.257638
          0.257638  0.0       0.0
          0.0       0.257638  0.0
        }
      }
    }
  }
}
</pre>


*/