Total atomization energies

J. M. L. Martin. BenchmarkAb Initio Calculations of the Total Atomization Energies of the First-Row Hydrides AHn (A = Li-F). Chem. Phys. Lett. 1997, 273, 98-106. 

Obtained from experimental dissociation energies and estimated total atomic energies, see text. c lAi state. 

Our philosophy was instead top-down . We decomposed the molecular TAE (total atomization energy TAEe at the bottom of the well, TAEo at absolute zero) into all components that can reasonably affect it at the kl/mol level. Then we carried out exhaustive benchmark calculations on each component separately for a representative training set of molecules. Finally, for each component separately, we progressively introduced approximations up to the point where reproduction of that particular component started deteriorating to an unacceptable extent. Thus, experimental data entered the picture only at the validation stage, not at the design stage. 

The W1 and W2 protocols for obtaining the total atomization energy (TAE) of a given molecule involve the following steps ... 

Table 2.5 Individual components (kcal/mol) in Wlh, Wl, and W2h total atomization energy cum heat of formation of benzene.a...

A third class of compound methods are the extrapolation-based procedures due to Martin [5], which attempt to approximate infinite-basis-set URCCSD(T) calculations. In the Wl method [16] calculations are performed at the URCCSD and URCCSD(T) levels of theory with basis sets of systematically increasing size. Separate extrapolations are then performed to determine the SCF, URCCSD valence-correlation, and triple-excitation components of the total atomization energy at... 

Table I Computed total atomization energy at 0 K of aniline and proton affinities at 298 K for protonation at different sites. Proton affinities for N-protonation of aniline. All values are given in kcal/mol.

The Wlc total atomization energy at 0 K of aniline, 1468.7 kcal/mol, is in satisfying agreement with the value obtained from heats of formation in the NIST WebBook 39), 1467.7 0.7 kcal/mol. (Most of the uncertainty derives from the heat of vaporization of graphite.) The various contributions to this result are (in kcal/mol) SCF limit 1144.4, valence CCSD correlation energy limit 359.0, connected triple excitations 31.7, inner shell correlation 7.6, scalar relativistic effects -1.2, atomic spin-orbit coupling -0.5 kcal/mol. Extrapolations account for 0.6, 12.1, and 2.5 kcal/mol, respectively, out of the three first contributions. 

In this section, we collect the experimental data which are required for the determination ofthe empirical electron correlation energy. In Table 1 the total atomic energies are... 

The hydrogen atom in the hydrogen-bonded complex is destabilized, and the destabilization, defined as the difference in total atomic energy between the hydrogen-bonded complex and the monomer, AE, is positive. 

In a different approach, one obtains Scn = 60.44 kcal/mol for the CN bond found in methylamine, from its total atomization energy and the energies of its CH and NH bonds. 

For the total atomization energy AHatom = 2 A//atom[C + 6 A7/atom[H] + A/7atom[OJ, we obtain... 

Consistent Basis Sets for the Calculation of Total Atomization Energies, Geometries, and Harmonic Frequencies. 

J.M.L. Martin, Ab initio total atomization energies of small molecules—Towards the basis set limit, Chem. Phys. Lett. 259 (5-6) (1996) 669-67S. 

The total atomic energy in this approximation is given by the sum of the tabulated three contributions. Note that it does not involve correlation energy. All entries are in relative values E/Eh ( Appendix A), The configurations correspond to a normal aufbau process and they coincide in most cases with those proposed for the ground states from experimental information. 

The total atomization energy is then 238 kcal. per mole, compared with the experimental value of 239. Note that in this 21% ionic compound, covalent and ionic contributions to the atomization energy are equal. 

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