Beryllium monohydride (BeH): Where we are now, after 86 years of spectroscopy

BeH is one of the most important benchmark systems for ab initio methods and for studying Born-Oppenheimer breakdown. However the best empirical potential and best ab initio potential for the ground electronic state to date give drastically different predictions in the long-range region between where the highest measurements have been made, and the dissociation energy; a region which is about similar to 1000 cm(-1) for (BeH)-Be-9, similar to 3000 cm(-1) for (BeD)-Be-9, and similar to 13 000 cm(-1) for (BeT)-Be-9. Improved empirical potentials and Born-Oppenheimer breakdown corrections have now been built in this work for the ground electronic states X(1(2)Sigma(+)) of all three isotopologues. The predicted dissociation energy for (BeH)-Be-9 from the new empirical potential is now in agreement with the current best ab initio prediction in all 5 digits of the former's precision, while the previous best empirical potential was in disagreement by 74 cm(-1). The previous best empirical potential predicted the existence of unobserved vibrational levels for all three isotopologues, and the current best ab initio study also predicted the existence of all of these levels, and 7 more in total. With the exception of two, the present empirical potential agrees with the existence of all of the ab initio potentials' extra levels not predicted by the earlier empirical potential. With one exception, all energy spacings between vibrational energy levels for which measurements have been made, are predicted with an agreement of better than 1 cm(-1) between the new empirical potential and the current best ab initio potential, but some predictions for unobserved levels are still in great disagreement, and the equilibrium bond lengths are different by orders of magnitude. (C) 2014 Elsevier Inc. All rights reserved.