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In this work we analyze how the choice of the active space in the Ž CASSCF the complete-active-space multiconfiguration self-consistent-field . Ž method and CASPT2 the second-order perturbation theory based on the . CASSCF reference wave function calculations affects the computed potential Ž . energy curves PECs for the intramolecular proton transfer reaction in the ground state and the two lowest lying singlet excited states of 1-amino-3propenal. As anticipated, the results revealed that, qualitatively, the proton transfer in the different states can be correctly described even by minimal active spaces, which include the orbitals involved in the electronic excitation of the considered state and the antibonding sigma orbital corresponding to the bond formed by the molecule with the migrating hydrogen atom. However, quantitatively, the relative energies of the two tautomers and the energy barriers computed at the CASSCF level change when the active space is increased, indicating importance of the dynamic electron correlation. Introducing the dynamic correlation effects via CASPT2 makes the calculated energy parameters more uniform among the different active spaces. The analysis suggested certain optimal active spaces for studying proton transfer reactions in systems similar to 1-amino-3-propenal. The PEC calculations for excited states showed that the results are sensitive to the molecular geometries used in the calculations, Correspondence to: L. Adamowicz; e-mail ludwik@ccit.arizona.edu Ž . Contractrgrant sponsor: Generalitat de Catalunga CIRIT ; contract grant number FIr96-05011.1 Contractrgrant sponsor: NSF and the Spanish DGICYT; contractrgrant number PB95-0762 ( ) Journal of Computational Chemistry, Vol. 20, No. 13, 1422]1431 1999 Q 1999 John Wiley & Sons, Inc. CCC 0192-8651 / 99 / 131422-10 CASSCF ]CASPT2 AND INTRAMOLECULAR PROTON TRANSFER REACTION particularly near the transition point. Q 1999 John Wiley & Sons, Inc. J Ž . Comput Chem 20: 1422]1431 1999 Ž .

A CASSCF-CASPT2 study of the excited-state intramolecular proton transfer reaction in 1-amino-3-propenal using different active spaces