Dual-level direct dynamics calculation of the ground-state proton transfer reaction is reported for the 7-azaindole(7AI)/carboxylic acid system. The reaction path was calculated and the two-dimensional potential energy surface scan was performed at various levels of theory. Only one transition state geometry was resolved in the ground state from the ab initio calculation. The zero-point corrected barrier for the reverse proton transfer was found to be small (similar to0.2 kcal/mol). The calculated reverse rate constant (1.45 x 10(12) s(-1)) at 300 K is significantly higher than the rate of tautomer emission (3.8 x 10(8) s(-1)) from the S-1(') excited state. (C) 2003 Elsevier Science B.V. All rights reserved.