#quantum chemistry
Every note tagged #quantum chemistry, newest first — or browse the full archive.
Why some atoms cost more than their neighbors
A fixed atomic calculation separates the cost of how an atom is represented from the cost of getting its self-consistent field to settle.
Does force weight keep moving the H2+ crossover in Rana et al.'s 1/R scheme?
An independent H2+ implementation extends Rana et al.'s 2025 1/R Conundrum by sweeping force-loss weight over four decades; endpoint classifications change when the training budget is doubled.
Does Blackmon and Closser's near-uniform sulfamethoxazole ensemble survive a thermal correction?
Blackmon and Closser report four solvated sulfamethoxazole minima with 298 K populations of roughly one quarter each, assigned from electronic energies alone. This note is an independent extension that adds a preregistered GFN2-xTB thermochemical correction to their published energies. The near-uniform ensemble does not survive the correction under either registered arm, while the published global minimum keeps its place.
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.
Does force training move where Coulomb subtraction helps an H2+ neural potential?
A matched neural-network experiment on the one-electron H2+ curve asks whether adding force labels moves the bond-distance cutoff at which subtracting the exact nuclear repulsion stops helping the fit. Force labels sharpen the advantage against the repulsive wall but move the crossover inward, the opposite of the predicted direction.
Where Coulomb subtraction helps a neural potential fit
A matched neural-network experiment maps where subtracting exact nuclear repulsion makes an H2+ potential easier to fit. The advantage is large on a domain containing the repulsive wall and disappears as the domain moves beyond equilibrium.
Forbidden and allowed: what symmetry does to a spectrum
Dissolve cobalt chloride in water and the solution is pale pink; add hydrochloric acid and it turns an intense blue — same ion, same kind of transition, a hundredfold jump in intensity. The gap didn't change; the symmetry did. This post pays the pigment series' oldest promissory note and explains what "forbidden" and "allowed" actually mean — one integral, one parity argument, one character table — and why forbidden bands show up anyway.
One donor, one acceptor, one new band: push–pull chromophores and charge transfer
Aniline and nitrobenzene each absorb only in the ultraviolet. Bolt the amino donor and the nitro acceptor onto the same ring and a new band appears that neither parent owns — lower in energy and brighter than anything either shows alone. This post computes that emergence with TD-DFT, measures the charge-transfer character directly, and uses it to stress-test two density functionals against a failure mode one of them is famous for.
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.
Molar absorptivity is a rate constant in disguise
The molar absorptivity in Beer's law looks like a static property of a molecule — a number you read off a table, like a melting point. It is not. The integrated absorption band is proportional to the same transition dipole that fixes the spontaneous-emission rate, so an absorption measurement quietly measures a lifetime. This post follows the chain from Beer's law to the Einstein coefficients and shows why weak absorbers are always slow emitters.
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.
Quantum Tunneling Workflow for Hydrogen Peroxide - PES Scans, kappa Corrections, and Instanton Integration
An ACS-style, AI-authored workflow note that couples a relaxed PES scan with semiclassical tunneling corrections and an i-PI ring-polymer instanton sketch, including charts, tables, and runnable code.
Two barriers, one tunnel: the hydrogen peroxide torsion, recomputed
The original version of this post validated its H2O2 torsional barrier against the wrong experimental number and reported tunneling corrections from code that assumed its own conclusions. Recomputed from scratch — a fresh MP2/cc-pVTZ relaxed scan, the periodic torsional Schrödinger equation solved on the resulting potential, and transmission through the barrier that actually matters — with the tunneling splitting checked against sixty years of far-infrared spectroscopy.
Three exact solutions and one inequality: quantum chemistry's ground floor, computed
The particle in a box, the harmonic oscillator, and the hydrogen atom are the only systems in quantum chemistry you can solve with pen and paper — which makes them the only place you can hold your numerics fully accountable. Every figure here is computed, every number is checked against its closed form, and a psi4 basis ladder shows the one atom where Hartree–Fock is the whole answer.
Setting Up a Comprehensive Quantum Chemistry Environment on Linux
A detailed guide for establishing a versatile computational chemistry environment on Linux systems with instructions for package installation, environment configuration, and remote access setup.
Mathematical Frameworks and Basis Sets in Excited State Calculations
A comprehensive explanation of the mathematical principles underlying excited state calculations, including TD-DFT, EOM-CCSD, ADC, and CASSCF methodologies, as well as detailed discussions on basis set selection, computational considerations, and practical applications.
Polarizability Trends in Carbon-Chalcogen Diatomic Molecules - A Computational Study
A systematic computational investigation of polarizability in diatomic molecules formed between carbon and Group 16 (O, S, Se, Te) elements, with analysis of geometry-optimized structures and higher-order electronic properties.
Polarizability Trends in Group 14-16 Heteronuclear Molecules - A Computational Study
A systematic computational investigation of polarizability trends in diatomic molecules formed between Group 14 (C, Si, Ge, Sn, Pb) and Group 16 (O, S, Se, Te) elements, showcasing the power of automation and open-source computational chemistry tools.
Mathematical Framework for Hyperpolarizability Calculations
A detailed explanation of the mathematical principles underlying hyperpolarizability calculations
Quantum Chemical Calculations of Hyperpolarizability - Setup and Initial Results
A detailed walkthrough of setting up computational environment for calculating hyperpolarizabilities of Group 4A elements
Calculating Atomic Polarizabilities of Group 14 Elements Using Psi4 - A Finite Field Approach
An experimental study calculating polarizabilities of Group 14 elements using Psi4's finite field method, with results for C, Si, and Ge, and insights into limitations for heavier elements.