7.10 Restricted Active Space Spin-Flip (RAS-SF) and Configuration Interaction (RAS-CI)

7.10.7 Examples

Example 7.98  Input for a RAS-2SF-CI calculation of three states of the DDMX tetraradical using RASCI1. The active space (RAS2) contains 4 electrons in the 4 singly occupied orbitals in the ROHF quintet reference. Natural orbital occupancies are requested.

$molecule
   0 5
   C      0.000000    0.000000    1.092150
   C     -1.222482    0.000000    0.303960
   C     -2.390248    0.000000    1.015958
   H     -2.344570    0.000000    2.095067
   H     -3.363161    0.000000    0.537932
   C     -1.215393    0.000000   -1.155471
   H     -2.150471    0.000000   -1.702536
   C      0.000000    0.000000   -1.769131
   C      1.215393    0.000000   -1.155471
   H      2.150471    0.000000   -1.702536
   C      1.222482    0.000000    0.303960
   C      2.390248    0.000000    1.015958
   H      2.344570    0.000000    2.095067
   H      3.363161    0.000000    0.537932
$end

$rem
   EXCHANGE        hf
   CORRELATION     rasci
   BASIS           6-31g
   UNRESTRICTED    false
   MEM_TOTAL       4000
   MEM_STATIC      100
   RAS_ROOTS       3
   RAS_ACT         4
   RAS_ELEC        4
   RAS_OCC         25
   RAS_SPIN_MULT   0
   RAS_NATORB      true
$end

Example 7.99  Input for a RAS-2IP-CI calculation of triplet states of F2 molecule using the dianion closed shell F2-2 as the reference determinant. RASCI1 code is used

$molecule
   -2 1
   F
   F  1  1.4136
$end

$rem
   EXCHANGE        hf
   CORRELATION     rasci
   BASIS           cc-pVTZ
   MEM_TOTAL       4000
   MEM_STATIC      100
   RAS_ROOTS       2
   RAS_ACT         6
   RAS_ELEC        10
   RAS_OCC         4
   RAS_SPIN_MULT   3
$end

Example 7.100  Input for a FCI/STO-3G calculation of water molecule expanding the RAS2 space to the entire molecular orbital set. RASCI code is used.

$molecule
   0 1
   O   0.000  0.000   0.120
   H  -0.762  0.000  -0.479
   H   0.762  0.000  -0.479
$end

$rem
   EXCHANGE        hf
   CORRELATION     rasci
   BASIS           sto-3g
   MEM_TOTAL       4000
   MEM_STATIC      100
   RAS_ROOTS       1
   RAS_ACT         7
   RAS_ELEC        10
   RAS_OCC         0
   RAS_SPIN_MULT   1
   RAS_DO_HOLE     false
   RAS_DO_PART     false
$end

Example 7.101  Methylene single spin-flip calculation using RASCI2

$molecule
   0 3
   C     0.0000000    0.0000000    0.0000000
   H    -0.8611113    0.0000000    0.6986839
   H     0.8611113    0.0000000    0.6986839
$end

$rem
   EXCHANGE       HF
   CORRELATION    RASCI2
   BASIS          cc-pVDZ
   AUX_BASIS      rimp2-cc-pVDZ
   UNRESTRICTED   false
   RAS_ACT_OCC    1    ! # alpha electrons
   RAS_ACT_VIR    1    ! # virtuals in active space
   RAS_ACT_DIFF   0    ! # set to 1 for odd # of e-s
   RAS_N_ROOTS    4
   SET_ITER       25   ! number of iterations in RASCI
$end

Example 7.102  Two methylene separated by 10 Å; double spin-flip calculation using RASCI2. Note that the S2 values for this case will not be uniquely defined at the triply-degenerate ground state.

$molecule
   0 5
   C     0.0000000     0.0000000    0.0000000
   H    -0.8611113     0.0000000    0.6986839
   H     0.8611113     0.0000000    0.6986839
   C     0.0000000    10.0000000    0.0000000
   H    -0.8611113    10.0000000    0.6986839
   H     0.8611113    10.0000000    0.6986839
$end

$rem
   EXCHANGE       HF
   CORRELATION    RASCI2
   BASIS          cc-pVDZ
   AUX_BASIS      rimp2-cc-pVDZ
   RAS_ACT_OCC    2    ! # alpha electrons
   RAS_ACT_VIR    2    ! # virtuals in active space
   RAS_ACT_DIFF   0    ! # set to 1 for odd # of e-s
   UNRESTRICTED   false
   RAS_N_ROOTS    8
   SET_ITER       25
$end

Example 7.103  RASCI2 calculation of the nitrogen cation using double spin-flip.

$molecule
   1 6
   N
   N   1  4.5
$end

$rem
   EXCHANGE           HF
   CORRELATION        RASCI2
   BASIS              6-31G*
   AUX_BASIS          rimp2-VDZ
   RAS_ACT_OCC        3   ! # alpha electrons
   RAS_ACT_VIR        3   ! # virtuals in active space
   RAS_ACT_DIFF       1   ! # for odd # e-s, cation
   UNRESTRICTED       false
   N_FROZEN_CORE      2
   N_FROZEN_VIRTUAL   2
   RAS_N_ROOTS        8
   SET_ITER           25
$end