Molcas Forum

Anugraha

Member

Registered: 2025-12-17

Posts: 22

CASSCF root 2 optimization: low CI weights

Dear OpenMolcas developers and users,

I have performed a CASSCF optimization targeting the second root .I am attaching the input and relevant parts of the output below.

& GATEWAY
 Title = TUCA 1 methanol pi pi star casscf(8,8) opt nofloat -0.82729 angstrom   
 RICD
 coord = t_uca_1meth_MP2_opt_cc_pvdz.xyz
 Numerical
 Group = z
 Basis set = aug-cc-pvdz
 Constraints
 r1 = bond H13 N12
 r2 = bond H13 O17
Values
 1.0 r1 - 1.0 r2 = -0.82729 angstrom
End of Constraints

>>> EXPORT MOLCAS_MAXITER = 2000

>>> DO WHILE

  &SEWARD  
  &RASSCF
   Fileorb=INPORB 
   symmetry = 1
   Levshift = 1.0
   Iterations = 200 100
   Spin = 1
    nActEl = 8 0 0
   Inactive = 37 4
   Ras2 = 0 8
   Staverage = 2
   Rlxroot = 2
    
  &SLAPAF
 
>>> END DO

printout of CI-coefficients larger than  0.05 for root  2
      energy=    -604.368288
      conf/sym  22222222     Coeff  Weight
             2  222ud000  -0.38744 0.15011
             3  222u0d00   0.43422 0.18854
             6  22u2d000   0.42946 0.18444
             7  22u20d00   0.14755 0.02177
             9  22u2000d   0.05291 0.00280
            11  2u220d00  -0.07761 0.00602
            18  22202000   0.39432 0.15549
            19  2220ud00   0.11601 0.01346
            20  22200200  -0.16112 0.02596
            28  22ud2000   0.10453 0.01093
            29  22udud00   0.24720 0.06111
            35  22ud0u0d  -0.05529 0.00306
            40  2u2d0200  -0.05936 0.00352
            44  2u2du00d   0.09075 0.00824
            58  22022000  -0.15819 0.02502
            60  22020200   0.05786 0.00335
            75  2ud20u0d   0.06762 0.00457
            88  20222000  -0.05338 0.00285
           118  22uudd00   0.07546 0.00569
           121  22uud00d   0.07547 0.00570
           124  2u2udd00   0.06555 0.00430
           155  22u0d200   0.05911 0.00349
           214  220u2d00   0.06575 0.00432
           243  2ududu0d   0.05138 0.00264
           321  022u0d20  -0.05408 0.00292
           480  02u2d020  -0.05228 0.00273
           657  02202020  -0.05008 0.00251

      Natural orbitals and occupation numbers for root  1
      sym 2:   1.950266   1.943336   1.909552   1.824726   0.181310   0.085729   0.045885   0.059197

      Natural orbitals and occupation numbers for root  2
      sym 2:   1.947751   1.910527   1.567887   1.109842   0.880296   0.446577   0.048566   0.088553

From the output, I observe that for root 2, multiple configurations contribute with relatively small CI weights, and there is no single dominant configuration. When I visualize the orbitals in Chemcraft, I have observed separate alpha and beta orbitals, which was not the case in my previous calculations.

Any insights on how to interpret these results and how to improve the calculation would be greatly appreciated.

Thank you
Anugraha

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,204

Re: CASSCF root 2 optimization: low CI weights

There's nothing wrong with having low CI weights. If the state is multiconfigurational, that's what it is. (Then again, the CI weights depend on the orbital representation. There might be another representation with a more compact CI expansion.) As for alpha/beta orbitals, if you have a singlet (or any state with M_S=0) they should be identical.