From a network of computed reaction enthalpies to atom-based thermochemistry (NEAT).

A simple and fast, weighted, linear least-squares refinement protocol and code is presented for inverting the information contained in a network of quantum chemically computed 0 K reaction enthalpies. This inversion yields internally consistent 0 K enthalpies of formation for the species of the network. The refinement takes advantage of the fact that the accuracy of computed enthalpies depends strongly on the quantum-chemical protocol employed for their determination. Different protocols suffer from different sources of error; thus, the reaction enthalpies computed by them have "random" residual errors. Since it is much more natural for quantum-chemical energy and enthalpy results, including reaction enthalpies, to be based on the electronic ground states of the atoms and not on the historically preferred elemental states, and since these two possible protocols can be converted into each other straightforwardly, it is proposed that first-principles thermochemistry should employ the ground electronic states of atoms. In this scheme, called atom-based thermochemistry (AT), the enthalpy of formation of a gaseous compound corresponds simply to the total atomization energy of the species; it is always positive, and it reflects the bonding strength within the molecule. The inversion protocol developed and based on AT is termed NEAT, which represents the fact that the protocol proceeds from a network of computed reaction enthalpies toward atom-based thermochemistry, most directly to atom-based enthalpies of formation. After assembling a database that consisted of 361 ab initio reactions and reaction enthalpies involving 188 species, collected from 31 literature sources, the following dependable 0 K atom-based enthalpies of formation, Delta(f)${H{{{\rm AT}\hfill \atop 0\hfill}}}$, all in kJ mol(-1), have been obtained by means of NEAT: H(2)=432.07(0), CH=334.61(15), NH=327.69(25), OH=425.93(21), HF=566.13(31), CO=1072.08(28), O(2)=493.51(34), CH(2)=752.40(21), H(2)O=918.05(20), HO(2)=694.53(32), CO(2)=1597.77(40), CH(3)=1209.64(29), NH(3)=1157.44(33), C(2)H(2)=1625.78(40), and CH(4)=1641.68(40), in which the uncertainty values given in parentheses represent 95 % confidence intervals. The average deviation of these values from the well-established active thermochemical tables (ATcT) values is a mere 0.25 kJ mol(-1), with a maximum deviation of 0.7 kJ mol(-1). This shows that the use of a large number of ab initio reaction enthalpies within a NEAT-type protocol has considerable advantages over the sequential utilization of the ab initio information.