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= Xi'an-CI =
<<TableOfContents(4)>>

{{{
   Xi’an-CI program generates Multi Reference SDCI (MRCISD) wavefunctions (including internal contracted MRCISD on several different level accuracy), Multi Reference Second Order Perturbation Theory (including multi-state NEVPT2 (MS-NEVPT2), Multi Reference Third Order Perturbation Theory (SS-NEVPT3), Static-Dynamic-Static Second Order Perturbation Theory (SDSPT2), Static-Dynamic-Static Configuration Interaction (SDSCI), Configuration Based Multi Reference Second Order Perturbation Theory (CB-MRPT2) and Configuration Based Multi Reference Third Order Perturbation Theory (CB-MRPT3). This program is based on hole-particle symmetry based and GUGA for the computation of CI matrix elements. The program can calculate several eigenvectors simultaneously.
   Xi’an-CI program is written by Zhenyi Wen, Yubin Wang, Zhengting Gan, Bingbing Suo and Yibo Lei (Institute of Modern Physics, Northwest University, China). 
}}}
== General keywords ==
=== nroot ===
{{{#!wiki
  State Number
}}}
Example:

=== Electron ===
{{{#!wiki
  CI effective electron Number without electrons of frozen MOs in traint module for MO integral transformation
}}}
Example:

=== Symmetry ===
{{{#!wiki
  Symmetry of the target state.
}}}
Example:

=== Spin ===
{{{#!wiki
  Spin multiplicity. 2S+1
}}}

=== core ===
{{{#!wiki
  Number of frozen orbitals in each irreps, which must be missing or set to zero in each irreps if it has frozen MOs in traint module.
}}}
Example:

=== Inactive ===
{{{#!wiki
  Number of inactive orbitals in each irreps.
}}}
Example:

=== Active ===
{{{#!wiki
  Number of active orbitals in each irreps.
}}}
Example:

Comment: 
{{{
  If the above keywords are not set. the mcscf module information will be used. 
}}}

=== Maxiter ===
{{{#!wiki
  Maximum iteration Number of MRCISD. The default value is 500.
}}}
Example:

=== Conv ===
{{{#!wiki
  set threshold for CI energy, CI vector and Residual vector of MRCISD, respectively. The default value is set as the following example.
}}}
Example:
{{{
Conv
1.d-8 1.d-4 1.d-8
}}}


=== FCCI ===
{{{#!wiki
   Default for internal contraction module. This keyword is set for Fully internal Contraction module of CSFs, reference CSFs are not contracted for MRCISD calculation, while perturbation theory calculation all CI subspaces are internally contracted.    
}}}
Example:
{{{
 FCCI
}}}

=== NICI ===
{{{#!wiki
   This keyword is set for one internal Contraction module of CSFs, only internal CI subspace are not contracted.    
}}}
Example:
{{{
 NICI
}}}


=== CWCI ===
{{{#!wiki
   This keyword is set for one internal Contraction module of CSFs, corresponding to mrcic in molpro program for Celani-Werner (CW) contraction, where only CI subspaces VV, DV, DDV and VD in hole-particle symmetry are not contracted.    
}}}
Example:
{{{
 CWCI
}}}

=== WKCI ===
{{{#!wiki
   This keyword is set for one internal Contraction module of CSFs, corresponding to mrci in molpro program for Werner-Knowles (WK) contraction, where only CI subspaces with two electron excitation to external spaces are contracted.    
}}}
Example:
{{{
 WKCI
}}}


=== SDCI ===
{{{#!wiki
   This keyword is set for one internal Contraction module of CSFs, the accuracy of which is more accurate than CWCI but less than WKCI. In contrast with WKCI module, two electron excitation from hole space and meanwhile one electron excitation to external space are also contracted.    
}}}
Example:
{{{
 SDCI
}}}

Comment:
{{{
  If no keyword is set for perturbation theory calculation in the following, xianci module will calculate MRCISD in default. 
}}}

=== NEVPT2 ===
{{{#!wiki
  set for SS-NEVPT2 and MS-NEVPT2 calculations, where each reference state expands a specific CI space.    
}}}
Example:
{{{
 NEVPT2
}}}

=== MR-NEVPT2 ===
{{{#!wiki
  set for SS-NEVPT2 and MS-NEVPT2 calculations, where all reference states expand only one multi-states CI space.    
}}}
Example:
{{{
 MR-NEVPT2
}}}

=== NEVPT3 ===
{{{#!wiki
  set for SS-NEVPT3 calculation, where each reference state expands a specific CI space.    
}}}
Example:
{{{
 NEVPT3
}}}

=== SDSPT2 ===
{{{#!wiki
  set for SDSPT2 calculation, where all reference states expand only one multi-states CI space.    
}}}
Example:
{{{
 SDSPT2
}}}

=== SDSCI ===
{{{#!wiki
  set for SDSCI calculation, where all reference states expand only one multi-states CI space.    
}}}
Example:
{{{
 SDSCI
}}}

=== NOLAN ===
{{{#!wiki
  set for SDSPT2 and SDSCI calculations, where no Lanczos wavefunctions are used to produce Ps wavefunction in SDSPT2 and SDSCI.   
}}}
Example:
{{{
 NOLAN
}}}

=== NDIMPS===
{{{#!wiki
  set for SDSPT2 and SDSCI calculations, where CASSCF wavefunctions are used to produce Ps wavefunction in SDSPT2 and SDSCI.   
}}}
Example:
{{{
 NDIMPS
  2   # two high-lying CASSCF wavefunctions are used to produce Ps wavefunction in SDSPT2 and SDSCI relative to reference wavefunctions.
}}}
Comment:
{{{
 If Keyword 'NDIMPS' are not set or set to zero and keyword 'NOLAN' are set, SDSPT2 or SDSCI has no Ps wavefunction. 
}}}