kcalmolHA

kcalmolHA is a nonstandard energy unit encountered in some computational chemistry and materials science literature, used to express interaction energies related to hydroxyapatite (HA) surfaces. It is not an officially sanctioned SI unit, and its exact meaning can vary between studies. In practice, kcalmolHA functions as a way to indicate that energies are referenced to HA, rather than to a generic amount of substance.

In many papers focusing on biomineralization, dental materials, or bone-material interfaces, kcalmolHA denotes energies normalized to the amount of hydroxyapatite present, such as per mole of HA formula units or per mole of surface sites. To compare with conventional reporting, authors often provide the same energy value in standard units, such as kcal/mol, and clarify the HA reference used. The specific HA model—including surface termination, protonation state, and particle size—must be specified to interpret the value correctly.

Calculation methods for energies expressed as kcalmolHA typically arise from molecular docking, molecular dynamics, or quantum mechanical calculations where a ligand, drug, peptide, or other species interacts with an HA surface or phase. Negative values indicate favorable (exothermic) interactions, while positive values indicate unfavorable interactions.

Limitations include the lack of standard definition and potential confusion when comparing across studies. Best practices encourage reporting both kcal/mol and kcalmolHA (when used), explicitly stating the HA reference, surface model, and simulation conditions to ensure clarity and facilitate cross-study comparisons.