Skip to content

5.3 Explicit solvation and the difference between interaction and free energy

These are unexecuted teaching inputs and starting models. Original diagrams are schematics, not calculated results. Validate version-specific syntax, licensed or authorized data, numerical convergence and the scientific model before using this workflow.

5.3.1 Model, units and provenance

Keep basis/potential files and executable versions traceable. Grid controls use Ry; common energy/force outputs use hartree and hartree/bohr. Read the unit in each output heading.

Shared inputs, conventions and evidence

Original schematic: Explicit solvation and the difference between interaction and free energy. No numerical results are claimed.
Original schematic: Explicit solvation and the difference between interaction and free energy. No numerical results are claimed.

5.3.2 Unexecuted inputs and explicit deltas

Use the accompanying instructions to identify the parent calculation and placement of every delta; a snippet is not automatically a standalone input. Preserve all blank-line and file-provenance requirements.

5.3.2.1 Input block 1

&KIND C
  ELEMENT C
  BASIS_SET DZVP-MOLOPT-GTH
  POTENTIAL GTH-PBE-q4
&END KIND

5.3.3 Worked investigation

Intuition. A solute in water changes both electronic interactions and the solvent configurations it can visit. One optimized cluster energy cannot represent a bulk solvation free energy. This case builds a neutral methanol-in-water sampling problem and defines what can be learned from snapshots before any rigorous free-energy method is attempted. Prerequisites: validated liquid-water preparation, a separately optimized neutral methanol geometry, and verified C/O/H basis/potential records.

Original model recipe. Replace one water molecule in a sufficiently large prepared water cell by methanol, then repack or remove overlaps while documenting the final number of molecules and concentration. Do not insert a molecule into occupied space and hope the thermostat fixes it. Add a carbon kind, after verifying its record:

See input block 1 above.

Keep total charge zero, use periodic XYZ electrostatics, and recalculate total electron count from all kinds. The coordinate file and concentration are required model choices, not hidden defaults. A small cell containing one solute represents a finite concentration, not infinite dilution.

Workflow. 1. Establish the isolated solute geometry and charge state independently. 2. Prepare at least two solvent arrangements around it. 3. Check static forces, then equilibrate under the selected DFT Hamiltonian and ensemble. 4. Define structural observables before production: solute–water O distance distributions, hydrogen-bond criteria with explicit distance/angle cutoffs, and solute conformational coordinates. 5. Save decorrelated snapshots for electronic analysis. 6. If computing snapshot interaction energies, state whether fragments retain the instantaneous geometry and whether surrounding solvent is included or removed; use a consistent ghost/basis protocol if required.

Interpretation. Snapshot interaction-energy averages omit entropy and generally are not solvation free energies. A hydrogen-bond count depends on the operational definition; report its sensitivity to modest cutoff changes. Residence times need a continuous/intermittent definition and trajectories long enough to observe exchanges. Solvation-shell convergence is not guaranteed by a pretty first-shell RDF; outer shells and periodic solute replicas can matter.

Tests and limits. Increase cell size or vary solute concentration, compare independent starts and block-average observables. Changing charge state invokes proton/electron reservoirs and additional electrostatic issues, not just one CHARGE keyword. Implicit solvation is an alternative model with its own cavity/dielectric conventions; do not combine arbitrary dielectric settings with an explicit box without defining the intended hybrid model.

Exercise. Write two distinct research claims supported by this workflow, such as a sampled coordination distribution and a snapshot polarization trend, and two claims it cannot establish, such as an absolute hydration free energy without an appropriate thermodynamic cycle. Plan the extra sampling needed for one unsupported claim.

Diagram. Solute surrounded by first and outer solvent shells, repeated cell boundaries and multiple snapshot cards. EN: finite concentration, local structure, ensemble average, free energy needs a path. Mark shells as illustrative.

5.3.5 Sources and further reading