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I have performed CASSCF calculations for a series of molecules with varying input orbitals. I have used UNO, UHF, ROHF and MP2 based natural orbitals to perform the CASSCF calculations. For my system I just wanted my input to be just pi-orbitals but the number of orbitals containing pi orbitals is different set of theory to form pi orbitals, with being less in number and least conjugated with MP2 based natural orbitals. When I am using MP2 based natural orbitals, the active space is turning out to be same for some molecules in the series. For example, I have an acene type of series and I am getting same active space say (8,8) for systems containing 6 benzene rings , 7 benzene rings and 8 benzene rings. Since the length of the system is increasing in a systematic fashion in terms of conjugation and the number of benzene rings, shouldn't the number of pi orbitals also increase in same manner ? But I am not able to see that, and therefore not able to justify the use of same active space for all. (For other cases like UNO, UHF the number of pi orbitals are increasing and also giving highly conjugated pi-orbitals in picture which is giving difficulty in convergence and for molecules big in the series exceeding the CASSCF limit.)

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