#electron correlation
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Does one C=C increment fit every alkene? Two preregistered tests of Witkowski and co-workers' correlation energy per bond
Witkowski, Śmiga, Hirata, Dral and Grabowski estimate molecular correlation energies as a sum of fitted bond-type increments and state that the assignment holds regardless of conjugation or geometry. This note is an independent reanalysis of their published tables plus a new coupled-cluster calculation on the four butene isomers their model cannot distinguish. Both preregistered verdicts came out inconclusive under the frozen decision rules; what survives is a systematic offset in the price of one bond swap and a measurable correlation split among isomers the model assigns identical energies.
How electron correlation survives a hydrogenation enthalpy subtraction
A worked CCSD(T)−HF calculation shows how large molecular correlation energies mostly cancel in reaction enthalpies, what survives that subtraction, and why the residual should not be assigned to one π bond.
How much does correlation really cost? The correlation gap in water, measured
The Hartree–Fock post drew its correlation-gap figure schematically. This post runs the actual calculations — RHF marched up a basis-set ladder to its limit, MP2 and CCSD(T) below it — and reports what electron correlation costs, in hartrees, for one bent molecule of water.
Hartree–Fock and the correlation gap: where the orbital energies come from
A water molecular-orbital diagram quotes orbital energies as if they were just there to be read off. This post derives the ground-state machinery — the Hartree–Fock equations, their self-consistent solution, Koopmans' theorem, and the correlation energy that the mean field leaves behind — that actually computes them.