/*! \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$, can be accomodated. Moreover, BCS wave function is implemented only for homogeneous systems, where the pair orbital depends only on a single vector, i.e., the distance of the two particles, \f$ \phi({\bf r}_i,{\bf r}_j) = \phi({\bf r}_i-{\bf r}_j) \f$. For description of more general cases see \ref Pfaff_wfDoc wave function. \section options Options \subsection reqopt Required
Option Type Description
PAIR_ORBITAL Section A section containing definition of one or several \ref Jastrow_groupDoc s that construct the pair orbital \f$ \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.
\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.
BCS
PAIR_ORBITAL { 
  JASTROW2
  GROUP { 
    TWOBODY { 
      COEFFICIENTS { 1.0 1.0  } 
    } 
    EEBASIS { 
      EE
      POLYPADE
      RCUT 12.1877
      BETA0 -0.8
      NFUNC 2
    }
  }
}
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.
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
        }
      }
    }
  }
}
*/