$CIS group                      required when CITYP=CIS                         
                                 required when CITYP=SFCIS                      
   The CIS method (singly excited CI) is the simplest way                       
to treat excited states.  By Brillouin's Theorem, a single                      
determinant reference such as RHF will have zero matrix                         
elements with singly substituted determinants.  The ground                      
state reference therefore has no mixing with the excited                        
states treated with singles only.  Reading the references                       
given in Section 4 of this manual will show the CIS method                      
can be thought of as a non-correlated method, rigorously so                     
for the ground state, and effectively so for the various                        
excited states.  Some issues making CIS rather less than a                      
black box method are:                                                           
    a) any states characterized by important doubles are                        
       simply missing from the calculation.                                     
    b) excited states commonly possess Rydberg (diffuse)                        
       character, so the AO basis used must allow this.                         
    c) excited states often have different point group                          
       symmetry than the ground state, so the starting                          
       geometries for these states must reflect their                           
       actual symmetry.                                                         
    d) excited state surfaces frequently cross, and thus                        
       root flipping may very well occur.                                       
The normal CIS implementation allows the use of only RHF                        
references, but can pick up both singlet and triplet                            
excited states. Nuclear gradients are available, as are                         
properties.  The CIS run automatically includes computation                     
of the dipole moments of all states, and all pairwise                           
transition dipoles and oscillator strengths.                                    
The spin-flip type of CIS is very similar to spin-flip TD-                      
DFT (the $TDDFT input contains more information about how                       
spin-flip runs select the target state's Ms by $CONTRL's                        
MULT value).  The reference state must be UHF or ROHF, with                     
MULT in $CONTRL at least 3.  The target states of the CIS                       
have one lower Ms, after one alpha spin in the reference is                     
flipped to beta.  Nuclear gradients are possible.                               
Solvent effects are not available for either CIS or SFCIS.                      
It is worthwhile to look at the $TDDFT input, which is a                        
very similar calculation.  The TD-DFT program offers the                        
possibility of recovering some of the correlation energy,                       
permits some solvent models, and can be used for MEX/CONICL                     
surface intersection searches.                                                  
The first six keywords are chemically important, while the                      
remainder are mostly technical.                                                 
NACORE = n Omits the first n occupied orbitals from the                         
           calculation (frozen core approximation).                             
           For CITYP=CIS, the default for n is the number                       
           of chemical core orbitals.                                           
           For CITYP=SFCIS, the default, which is also the                      
           only possibility, is 0.                                              
NSTATE =   Number of states to be found (excluding the                          
           reference state).  No default is provided.                           
IROOT  =   State for which properties and/or gradient will                      
           be calculated.  Only one state can be chosen.                        
           The reference state is referred to as 0, and in                      
           the case of CITYP=SFCIS, might have a higher                         
           energy than some of the NSTATE target states.                        
CISPRP =   Flag to request the determination of CIS level                       
           properties, using the relaxed density.  Relevant                     
           to RUNTYP=ENERGY jobs, although the default is                       
           .FALSE. because additional CPHF calculation will                     
           be required.  Properties are an automatic by-                        
           product of runs involving the CIS or SFCIS                           
           nuclear gradient.                                                    
HAMTYP =   Type of CI Hamiltonian to use, if CITYP=CIS.                         
       =   SAPS spin-adapted antisymmetrized product of                         
                the desired MULT will be used (default)                         
       =   DETS determinant based, so both singlets and                         
                triplets will be obtained.                                      
MULT   =   Multiplicity (1 or 3) of the singly excited                          
           SAPS (the reference can only be singlet RHF).                        
           Only relevant for SAPS-based CITYP=CIS run,                          
           as SFCIS controls the Ms for target states by                        
           the value of MULT in $CONTRL.                                        
                    - - - - - - - - - - - -                                     
DIAGZN =   Hamiltonian diagonalization method.                                  
       =   DAVID use Davidson diagonalization.  (default)                       
       =   FULL  construct the full matrix in memory and                        
                 diagonalize, thus determining all states                       
                 (not recommended except for small cases).                      
DGAPRX =   Flag to control whether approximate diagonal                         
           elements of the CIS Hamiltonian (based only on                       
           the orbital energies) are used in the Davidson                       
           algorithm.  Note, this only affects the rate of                      
           convergence, not the resulting final energies.                       
           If set .FALSE., the exact diagonal elements are                      
           determined and used.  Default=.TRUE.                                 
NGSVEC =   Dimension of the Hamiltonian submatrix that is                       
           diagonalized to form the initial CI vectors.                         
           The default is the greater of NSTATE*2 and 10.                       
MXVEC  =   Maximum number of expansion basis vectors in the                     
           iterative subspace during Davidson iterations,                       
           before the expansion basis is truncated.  The                        
           default is the larger of 8*NSTATE and NGSVEC.                        
NDAVIT =   Maximum number of Davidson iterations.                               
DAVCVG =   Convergence criterion for Davidson eigenvectors.                     
           Eigenvector accuracy is proportional to DAVCVG,                      
           while the energy accuracy is proportional to its                     
           square.  The default is 1.0E-05.                                     
CHFSLV =   Chooses type of CPHF solver to use.                                  
       =   CONJG selects an ordinary preconditioned                             
                 conjugate gradient solver.  (default)                          
       =   DIIS  selects a diis-like iterative solver.                          
RDCISV =   Flag to read CIS vectors from a $CISVEC input                        
           group in the input file.  Default is .FALSE.                         
MNMEDG =   Flag to force the use of the minimal amount of                       
           memory in construction of the CIS Hamiltonian                        
           diagonal elements.  This is only relevant when                       
           DGAPRX=.FALSE., and is meant for debug purposes.                     
           The default is .FALSE.                                               
MNMEOP =   Flag to force the use of the minimal amount of                       
           memory during the Davidson iterations. This is                       
           for debug purposes. The default is .FALSE.                           
$CISVEC group     required if RDCISV in $CIS is chosen                          
This is formatted data generated by a previous CIS run, to                      
be read back in as starting vectors.  Sometimes molecular                       
orbital phase changes make these CI vectors problematic.                        

generated on 7/7/2017