CO Dimer: New Potential Energy Surface and Rovibrational Calculations

The spectrum of CO dimer was investigated
by solving the rovibrational
Schrödinger equation on a new potential energy surface constructed
from coupled-cluster ab initio points. The Schrödinger equation
was solved with a Lanczos algorithm. Several 4D (rigid monomer) global
ab initio potential energy surfaces (PESs) were made using a previously
reported interpolating moving least-squares (IMLS) fitting procedure
specialized to describe the interaction of two linear fragments. The
potential has two nonpolar minima giving rise to a complicated set
of energy level stacks, which are very sensitive to the shapes and
relative depths of the two wells. Although the CO dimer has defied
previous attempts at an accurate purely ab initio description our
best surface yields results in good agreement with experiment. Root-mean-square
(rms) fitting errors of less than 0.1 cm–1 were
obtained for each of the fits using 2226 ab initio data at different
levels. This allowed direct assessment of the quality of various levels
of ab initio theory for prediction of spectra. Our tests indicate
that standard CCSD­(T) is slow to converge the interaction energy even
when sextuple zeta bases as large as ACV6Z are used. The explicitly
correlated CCSD­(T)-F12b method was found to recover significantly
more correlation energy (from singles and doubles) at the CBS limit.
Correlation of the core–electrons was found to be important
for this system. The best PES was obtained by extrapolation of calculations
at the CCSD­(T)­(AE)-F12b/CVnZ-F12 (n = 3,4) levels. The calculated energy levels were compared to 105 J ≤ 10 levels from experiment. The rms error for
68 levels with J ≤ 6 is only 0.29 cm–1. The calculated energy levels were assigned stack labels using several
tools. New stacks were found. One of them, stack y1, has an energy lower than many previously known stacks
and may be observable.