/*! \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$. For description of more general cases see \ref Pfaff_wfDoc wave function. \section options Options \subsection reqopt Required
Option | Type | Description |
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PAIR_ORBITAL | Section | A section containing definition of one or several \ref Jastrow_groupDoc s that construct the pair orbital \f$ \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. |
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 } } } } }*/