Total geometry optimizations are reported for Cr(CO),, HMn(CO),, Fe(CO),, (C,H,)CT(CO)~. A variety of basis sets were examined, and, based on the results, a relatively compact and accurate basis set is proposed. The differences between the calculated and experimental metal-carbonyl, metal-benzene, and metal-nitrosyl bond distances average 0.03, 0.08, and 0.07 A, respectively. Calculated metal-cyclopentadienyl bond lengths were found to be an average of 0.15 8, longer than experimental bond lengths. Addition of electron correlation at the perfect pairing GVB level reduced the average difference in the metal-cyclopentadienyl bond length to 0.08 8,.Ni(CO)4, Cr(C6H6)29 Fe(C5H5),, Ni(C4H4)29 Cr(NO),, (CsHs)Mn(CO)3, and