<div dir="ltr"><div>Dear all, </div>Hi, I am now trying to compute the energy difference between high spin state and low spin state of octahedral metal complex with CAS level. <div><br></div><div>The problem that I am struggling is that the HF method does not give a proper configuration even in singlet state. The d orbitals are completely occupied and also some ligand orbitals are above them. It is difficult to generate the electronic structure with the correct configuration with HF. In this case, may I set the active space of CAS as fully occupied orbitals with 6 electrons? or is there any other way to make the correct electronic configuration with HF? </div><div><br></div><div>The case that I am trying is quite simple one. Here is my target geometry</div><div><br></div><div><div>13</div><div><br></div><div>Fe    0.000000   0.000000   0.000000</div><div>C     0.000000   0.000000   2.003402</div><div>C    0.000000   2.003402   0.000000</div><div>C    -2.003402   0.000000   0.000000</div><div>C    0.000000  -2.003402   0.000000</div><div>C    2.003402   0.000000   0.000000</div><div>C    0.000000   0.000000  -2.003402</div><div>O    0.000000   3.187907   0.000000</div><div>O    3.187907   0.000000   0.000000</div><div>O    0.000000   0.000000   3.187907</div><div>O    -3.187907   0.000000   0.000000</div><div>O    0.000000  -3.187907   0.000000</div><div>O    0.000000   0.000000  -3.187907</div></div><div><br></div><div><br></div><div>Sincerely </div><div>Sunghwan Choi</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br><div><div><br></div></div></div></div>