PSU CHEM 408 - Calculation of Molecular Binding Energies

Unformatted text preview:

FEATURE ARTICLEA Road Map for the Calculation of Molecular Binding EnergiesThom H. Dunning, Jr.EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,†Richland, Washington 99352ReceiVed: April 18, 2000; In Final Form: July 20, 2000During the past decade dramatic progress has been made in calculating the binding energies of molecules.This is the result of two advances reported in 1989: an accurate method for solving the electronic Schro¨dingerequation that is applicable to a broad range of moleculessthe CCSD(T) methodsand families of basis setsthat systematically converge to the complete basis set limitsthe correlation consistent basis sets. The formerprovides unprecedented accuracy for the prediction of a broad range of molecular properties, including molecularbinding energies. The latter provides a means to systematically approach the complete basis set limit, i.e., theexact solutions of approximations to the Schro¨dinger equation. These two advances combined with a thoroughanalysis of the errors involved in electronic structure calculations lead to clear guidelines for ab initiocalculations of binding energies, ranging from the strong bonds derived from chemical interactions to theextremely weak binding due to dispersion interactions. This analysis has also led to surprises, e.g., it hasshown that the Møller-Plesset perturbation theory is unsuitable for calculation of bond energies to chemicalaccuracy, i.e., with errors of less that 1 kcal/mol. This applies whether one is interested in absolute bondenergies or relatiVe bond energies. Although the analysis presented here is focused on the calculation ofmolecular binding energies, this same approach can be readily extended to other molecular properties.1. IntroductionThe concept of chemical bonds and the determination of bondenergies are central to chemistry. The making and breaking ofchemical bonds in molecules governs the behavior of manyprocesses important to our modern world, from the productionof energy and pollutants in an automobile engine to the catalyticprocesses that convert raw materials into materials of value tosociety. Weaker molecular interactions are also important.Hydrogen bonds play a critical role in a wide range of chemicalprocesses, especially biochemical processes. Both inter- andintramolecular forces determine the properties of polymers, anda wide range of materials has been developed by varying theseinteractions in a systematic manner. Obtaining a detailedunderstanding of molecular interactions and molecular bindingenergies is one of chemistry’s “Grand Quests.”With the discovery of the mathematical equation governingthe behavior of atoms and molecules in the mid-1920sstheSchro¨dinger equationsthe pathway was opened for calculatingmolecular binding energies from first principles. In fact,physicists immediately set about computing the binding energyof H2with great success. This work not only provided evidencesupporting the radical new quantum mechanics, but was the firstsuccessful prediction of a chemical bond energy. Unfortunately,what was possible for H2was not possible for other molecules,and, as far as the rest of chemistry was concerned, the commentby P. A. M. Dirac in 19291held:“The underlying physical laws necessary for the mathematicaltheory of a large part of physics and the whole of chemistryare thus completely known, and the difficulty is only that theexact application of these laws leads to equations much toocomplicated to be soluble.”Despite this difficulty, scientists such as Pauling, Mulliken, andothers used the framework provided by quantum mechanics todiscover the general laws which govern the structure andenergetics of molecules. This work had an enormous impacton chemistry and later led to the award of Nobel Prizes to thesetwo individuals.In his classic book, The Nature of the Chemical Bond,Pauling2stated that “there is a chemical bond between two atomsor groups of atoms in case that the forces acting between themare such as to lead to the formation of an aggregate withsufficient stability to make it convenient for the chemist toconsider it as an independent system.” This definition is stillvalid today, although what is considered an “independentsystem” is much different than in Pauling’s time. Through thedevelopment of sophisticated synthetic techniques and sensitivemeasurement technologies, experimental physical chemists haveprepared and characterized a wide range of weakly boundmolecular complexes. This is nowhere better illustrated than inthe recent report by Giese, Gentry, and co-workers3(see alsoref 4) of the synthesis and characterization of the helium dimersa molecule that is bound by only 1 milliKelvin (0.7 cm-1, 0.002kcal/mol). This work by experimental physical chemists hasgreatly increased our understanding of the full range ofmolecular interactions.†The Pacific Northwest National Laboratory is operated for the U.S.Department of Energy by Battelle Memorial Institute under Contract No.DE-AC06-76RLO 1830.9062 J. Phys. Chem. A 2000, 104, 9062-908010.1021/jp001507z CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 09/15/2000In the present article, we will consider all of the types ofinteractions that can give rise to a stable molecule or molecularcomplex. Specifically, four types of interactions will beconsidered.Chemical interactions, which result from the intimatesharing of electrons between the two atoms involved in the bond.Chemical bonds represent the strongest molecular interactions,and their strengths vary from tens to hundreds of kcal/mol,although most fall in the range of 75 to 150 kcal/mol.Hydrogen-bond interactions, which result from the sharingof a hydrogen atom between two other atoms. The strengths ofhydrogen bonds vary from a few kcal/mol to tens of kcal/moland are typically weaker than chemical bonds by a factor of 10or more.Electrostatic interactions, which arise from the classicalelectrostatic interactions between the multipole moments ofmolecules. In these systems, which we shall collectively referto as “weakly bound molecules,” binding energies range froma fraction of a kcal/mol to a few kcal/mol, another order ofmagnitude weaker than hydrogen bonds.Dispersion interactions, which result from the instantaneouscorrelation between the fluctuations in the electronic chargeclouds of the two interacting systems. Binding energies arisingsolely from dispersion interactions range from hundredths of


View Full Document

PSU CHEM 408 - Calculation of Molecular Binding Energies

Download Calculation of Molecular Binding Energies
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Calculation of Molecular Binding Energies and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Calculation of Molecular Binding Energies 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?