Enter an alkane — a chain, a branched skeleton, or a ring — and get the OPLS-UA energy of each distinct conformer, geometry-optimized. The relative energies that decide which shapes a flexible molecule actually adopts.
Many alkanes have multiple shapes. Each carbon–carbon bond in a chain can sit anti (stretched straight) or gauche (kinked), and a ring can pucker into a chair, a twist-boat, and beyond. Those choices define a family of conformers, and how much energy each one costs governs everything from a liquid’s density to how a drug-like molecule folds against its target.
This calculator takes any alkane — straight-chain, branched, or cyclic — generates its conformers with RDKit, and energy-minimizes each with the group’s OPLS-UA force field, the united-atom model where every CH2 and CH3 is a single interaction site. It reports each distinct conformer’s energy relative to the most stable one, in kcal/mol. The engine reproduces the conformer energies in Table 3 of the group’s 2026 OPLS-UA paper — cyclohexane, methylcyclohexane, cycloheptane, cyclooctane and the n-alkanes all to within ~0.25 kcal/mol.
Enter an alkane as a SMILES string — C’s for carbons, parentheses for branches, ring-closure digits for rings. Then see the relative conformer energies.
| Conformer | ΔE (kcal/mol) | # times found |
|---|
Each row is one distinct energy-minimized conformer, ranked by energy. The name gives its shape: for chains, the backbone pattern (a = anti/straight, g⁺ / g⁻ = gauche/kinked ±) — so hexane’s aag⁺ is anti–anti–gauche; for rings, the pucker — chair, boat, twist-boat, crown, boat-chair, twist-chair, envelope, and so on — with any substituent on a six-membered chair noted equatorial or axial, and fused rings (decalin) as cis or trans. The angles beneath each name are its defining torsions (backbone dihedrals for chains, ring torsions for rings), in degrees. ΔE is from the global minimum; # times found is how many sampled 3D conformers relaxed into that energy well.
Each conformer’s energy is the OPLS-UA sum of bond stretching, angle bending, torsional (twisting) energetics, and Lennard-Jones nonbonded interactions — with 1,4 interactions scaled by ½, as in the force field. Ring atoms carry the paper’s ring bonded parameters. Every structure is fully geometry-optimized (L-BFGS on the analytic gradient) before its energy output.
Conformers are generated with RDKit’s ETKDG, which samples chain rotamers and ring puckers (chair, twist-boat, and higher-ring forms). Each is minimized with OPLS-UA, and structures that relax to the same energy are folded into one conformer. Alkanes up to twelve carbons — straight-chain, branched, and cyclic — are supported.
Validation: the engine reproduces Table 3 of the 2026 OPLS-UA paper — the n-alkanes, cyclohexane (6.70), methylcyclohexane (1.83 vs 1.81), cycloheptane (4.92), cyclooctane (0.85), and 1,2-dimethylcyclohexane (1.20 vs 1.17) all within ~0.25 kcal/mol. A tertiary-carbon improper term recovers the correct anti/gauche ordering for branched cases like 2,3-dimethylbutane (gauche +0.20 vs the paper's +0.10).
conformer calculations run since 2026
Live usage — calculations are tallied by country using an offline geolocation database; no IP addresses are stored. Map: simple-world-map, CC BY-SA 3.0.
Jorgensen, W. L.; Tirado-Rives, J. Free Energies of Solvation in Cyclohexane and Water: United-Atom vs All-Atom. J. Chem. Theory Comput. 2026, 22, 7581–7593. doi:10.1021/acs.jctc.6c00989