The adiabatic potential surface of HCO obtained by the single-referenc
e ACPF method is quantitatively described using re-derived formula bas
ed on the treatment used for Jahn-Teller systems. The harmonic force c
onstant related to the bending vibration is negative with a lower abso
lute value than the corresponding vibronic constant. An insufficient n
umber of potential constants related to bending vibrations may lead to
qualitatively incorrect results. (C) 1998 Elsevier Science B.V.