Partial Optimization

[Yubo Fan <yubofan at.at guomai.sh.cn>]

 Hi, everyone:
 I am doing the geometry optimization of some rather big molecules. Can I
 split these molecules into some small pieces and optimize them
 seperately? For example, methyl(4-nitrophenyl)octadecylamine
 (C18H37N(CH3)C4H4NO2, MNPOA), I want to optimize the geometry of
 ethylmethyl(4-nitrophenyl)amine (EMNPA) by B3LYP/6-31G*, then change
 ethyl into octadecyl. Are there some methods in G94W to fix all bond
 lengths, bond angles and dihedral angles in EMNPA but the ethyl, and
 change the ethyl into octadecyl? Then do the geometry optimization of
 NMPOA by PM3 or other semi-empirical methods.
 With best regards
 Yubo Fan
 --
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 Yubo Fan                         Email: yubofan at.at guomai.sh.cn
 Organic Synthesis Lab                   yubofan at.at fudan.edu.cn
 The Department of Chemistry
 Fudan University                 Phone: 8621-65648139
 No. 220 Handan Road              Fax:   8621-65641740
 Shanghai, 200433
 P. R. China
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