CASSCF+CASPT2 for Vanadium Cation： remarks and summary

V+(H2O)：

A) Quintet

symmetry：C2V [1-A1; 2-B1; 3-B2; 4-A2];     CASSCF(4, 9)

 {casscf
   close,8,3,3,0
   occ,12,5,5,1
   wf,32,4,4;state,10
   wf,32,1,4;state,10
   wf,32,2,4;state,10
   wf,32,3,4;state,10
   natorb,ci,print}

A1:  close [for 8 pairs] : 1s [V]; 2s[V]; 1s[O]; 2py[V]; 3s[V]; 3py[V]; 2s[O]; O-H sigma bond (2py-O：1s-H)

A1:  [9-12] : 9-[d2+(V): dx2-y2:-0.206947]; 10-[d0(V): dz2: -0.207388]; 11-[4s(V): -0.207217]; 12-[4py(V): -0.087493]


B1:  close [3 pairs] : 2px[V]; 3px[V]; 2px[O]

B1: [4-5]: 4-[3d2-(V):dxy:-0.209038]; 5-[4px(V): -0.12012]


B2: close [3 pairs] : 2pz[V]; 3pz[V]; 2pz[O]

B2:[4-5]:4-[d1-(V):dxz: -0.199839]; 5-[4pz(V): -0.111116]


A2: close 0

A2: 1-[3d1+(V):dyz: -0.218392]


B) Triplet

If calculating the triplet state only including the electronic state or configuration state with spin=2, then we can not get the correct active space in which the active orbitals are: the five 3d orbital，one 4s orbital, and three 4p orbital of vanadium

Only if we combine the spin=4 configuration state (quintet state) to calculate the triplet state,  then we can get the correct active space.

 Geometry={
 4
 V+(H2O)-t.dat
 V                  0.00000000    0.00000000    0.66236305
 O                  0.00000000    0.00000000   -1.40863240
 H                  0.00000000    0.77937774   -1.98264542
 H                  0.00000000   -0.77937774   -1.98264542
}

 spherical
 basis=cc-pvtz


 {casscf
  close,8,3,3,0
  occ,12,5,5,1
  wf,32,1,4;state,6
  wf,32,2,4;state,6
  wf,32,3,4;state,6
  wf,32,4,4;state,6
  wf,32,1,2;state,6
  wf,32,2,2;state,6
  wf,32,3,2;state,6
  wf,32,4,2;state,6
  natorb,ci,print}


 {rs2,shift=0.3,thrvar=1.0d-8,thrden=1.0d-8;maxit,100;
  wf,32,1,4
  state,1,1}
 {rs2,shift=0.3,thrvar=1.0d-8,thrden=1.0d-8;maxit,100;
  wf,32,4,4
  state,1,1}
 {rs2,shift=0.3,thrvar=1.0d-8,thrden=1.0d-8;maxit,100;
  wf,32,1,2
  state,1,1}
 {rs2,shift=0.3,thrvar=1.0d-8,thrden=1.0d-8;maxit,100;
  wf,32,4,2
  state,1,1}


put,molden,test1.molden

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V+O：
A) Triplet
B) Singlet
V+OH：
A) Quartet
B) Doublet



V+(H2O)2：

A) Quintet

Symmetry :D2h : 1-Ag; 2-B3u; 3-B2u; 4-B1g; 5-B1u; 6-B2g; 7-B3g; 8-Au      CASSCF(4, 9)



1Ag:  close [for 6 pairs] : 1s [V]; 2s[V]; 1s[O];  3s[V]; 2s[O]; 2pz(O)-1s(H) [sigma bond]


1Ag[7-9]
7-[4s(V)+3d0:3dz2(V)]; 8-[3d2+:3dx2-y2(V)]; 9-[3d0:3dz2(V)];


2B3u:  close [3 pairs] : 2px[V]; 3px[V]; 2px(O)-1s(H) [sigma bond];

2B3u: [4]



3B2u: close [3 pairs] : 2py[V]; 3py[V]; 2py(O) [lone pair]

3B2u: [4]


4B1g: close 0
4B1g: 1-[3d2-:3dxy(V): -0.204883]


5B2u: close [5 pair]: 1s[O]; 2pz[V]; 3pz[V]; 2s[O]; 2pz[O]-1s[H] [sigma bond]

5B2u[6]


6B2g: close [1pair]: 2px[O]-1s[H] [sigma bond]
6B2g:[2]
2-[3d1+:3dxz(V):-0.171353]

7B3g: close [1 pair] 2py[O] [lone pair]
7B3g:[2]
2-[3d1-:3dyz(V):-0.133280]

8Au: close [0 pair]

B) Triplet : D2H

1Ag[close 6]: 1s[V]; 2s[V]; 1s[O]; 3s[V]; 2s[O]; sigma O-H [2pz(O)-1s(H)]
7-

2B3u[close 3]: 2px[V]; 3px[V];  sigma O-H [2px(O)-1s(H)]


3B2u[close 3]: 2py[V]; 3py[V]; non bond [2py(O)]

4B1g[close 1]: 1-[d2-:dxy]

5B1u[close 5 ]: 1s[O]; 2pz[V]; 3pz[V]; 2s[O]; sigma O-H [2pz(O)-1s(H)]

6B2g[close 1]: sigma O-H [2px(O)-1s(H)]

7B3g[close 1]: non bond [2py(O)]

8Au[close 0]





V+(H2O)3

A) Quintet : Symmetry Cs: A'; A'' 

1A'[close 19]

1-1s[V]; 2-2s[V]; 3-1s[O1]; 4-1s[O3]; 5-1s[O2]; 6-2py[V]; 7-2px[V]; 8- 3s[V]; 9-3py[V]; 10-3px[V]; 11-2s[O1]; 12-2s[O3]; 13-2s[O2]; 14-sigma bond [2px(O1)-1s(H1)]; 15-sigma bond []

2A''[close 5]