Tutorials

Start with a small calculation, learn how to inspect its result, then choose an electronic-structure or nuclear-motion example. Each lesson below links to its complete input and explanation. The method links lead to the References page so that the calculation and its scientific basis can be read together.

Before you start

1. Follow Installation and use one installed OpenQP environment. Check the version recorded in the calculation log.
2. Create an empty local working directory for each lesson. Save the .oqp input and any linked geometry, basis or force-field files there.
3. Start with two OpenMP threads. A thread request is subject to the installed edition's limits; this tutorial does not change licence or MPI policy.
4. Keep the input and complete log together. A successful process exit alone does not establish electronic convergence or a physically correct state.

Lesson 1 — water energy, orbitals and charges

Open Water: energy, orbitals and charges, then download its input into your working directory. The coordinates are included; no separate XYZ is needed.

openqp water-scf.oqp --omp 2

Read water-scf.log in this order:

Check	What to establish
Calculation identity	The requested PBE0 functional, 6-31G* basis, charge and reference are the ones actually used.
SCF convergence	The electronic iteration converged; an iteration limit is not a converged result.
Total energy	Compare the final total in Hartree with the value in the worked example, using the same input and numerical settings.
Orbitals	Identify HOMO and LUMO and their spatial character. Their orbital-energy gap is not an optical excitation energy.
Properties	Dipole and Mulliken charges were explicitly requested. Their absence is not a physical zero.

Change one setting at a time, retain a separate log, and explain the resulting quantity before comparing numbers. The published worked-example values are reference examples, not a new validation run of a future release.

Scientific background: HF/DFT, PBE0 and other functionals, and basis sets.

Lesson 2 — an equilibrium geometry is more than a small gradient

Follow Formaldehyde geometry and vibrations. Run optimization first, retain the final geometry, then calculate frequencies at that geometry. Inspect the number and character of imaginary modes before calling a stationary point a minimum. Zero-point and thermal corrections are separate quantities from the electronic energy.

Continue with HCN transition state and barrier when you can distinguish a minimum from a first-order saddle. Follow the reaction coordinate to check which minima the saddle connects.

Background: optimization and reaction paths and vibrations.

Lesson 3 — distinguish orbital gaps, response energies and state labels

Start with Formaldehyde UV absorption for conventional TDDFT. Then read Twisted ethylene with MRSF and SF-TDDFT and spin contamination.

Record the ground/reference energy, the physical state label, the excitation energy and oscillator strength separately. MRSF uses an internal high-spin reference: its first physical singlet response state is S0. Conventional TDDFT uses a different reference/state convention. Do not compare arrays solely by position or identify optical transitions from Kohn–Sham orbital differences.

Background: linear-response TDDFT, spin-flip theory, and MRSF-TDDFT.

Lesson 4 — choose a correlation treatment

Compare water MP2 and CCSD(T) for correlation energy, then H2 dissociation and LiH CASSCF/CASPT2 versus FCI for changing multiconfigurational character. Record the reference, active space, root and perturbative correction separately. A more elaborate method name does not by itself establish that a particular geometry or state is well described.

Background: MP2, coupled cluster, and multiconfigurational methods.

Advanced lessons — dynamics, embedding and properties

Goal	Lesson	Prerequisite checks
Ground-state dynamics	Methanol MD	Stable electronic convergence, timestep, ensemble and energy behaviour.
Surface hopping	Ethylene NAMD	Supported coupling scheme, state continuity, restart and conservation criteria.
Solvent embedding	Formaldehyde QM/MM	Download all PDB/XML assets; verify atom selection, topology and embedding model.
Spin–orbit coupling	Formaldehyde SOC	Separate spin manifolds, matrix/eigenvalue meaning and cm^-1 units.
Nuclear magnetic shielding	Benzene NMR	Gauge, shielding reference and shielding versus chemical shift.
Python analysis	State analysis	Correct units and physical state identity in exported results.

See the complete worked-example collection and method reference directory. Experimental or optional routes retain the limitations stated on their pages; a tutorial link does not add support to an older or differently built engine.

When results differ

Compare the complete input, engine version, basis convention, convergence thresholds, geometry, charge/reference, requested state and units first. Retain the full log and any required auxiliary files when reporting a problem. Use Troubleshooting and the keyword reference before changing several settings.