The following are preferred APBS units.


Many quantities are scaled by Avogadro’s number, the number of atoms in a mole (mol), \(N_A = 6.02214076 \times 10^{23} \, \text{mol}^{-1}\).


The preferred unit of length is the ångström (Å), equal to 10-10 meters.


The preferred unit of volume is Å3, equal to 10-27 liters.

Density and concentration

The preferred unit of density is number per Å3, corresponding to a concentration of approximately 1660.5391 mol L-1 or molar (M).


The preferred unit of temperature is Kelvin (K).


The preferred unit of charge is \(e_c = 1.602176634 \times 10^{−19} \, \text{C}\). The following number is often useful: \(N_A e_c = 9.64853321233 \times 10^4 \, \text{C} \, \text{mol}^{-1}\).


The preferred unit of energy is \(k_B \, T\) or \(R \,T\) where

  • Boltzmann’s constant: \(k_B = 1.38064852 \times 10^{-23} \, \text{J} \, \text{K}^{-1}\)
  • Gas constant: \(R = N_A k_B = 8.31446261815324 \text{J} \, \text{K}^{-1} \, \text{mol}^{-1}\)
  • Temperature: \(T\)

If \(T \approx 298 \, \text{K}\), then \(R\, T \approx 2.49 \, \text{kJ}\).

Surface tension

The preferred unit for surface tension is kJ mol-1 Å-2. Values for the surface tension of water in these models often range from 0.105 to 0.301 kJ mol-1 Å-2. [1] However, these values can vary significantly depending on the model used. [2] [3]


The preferred unit for pressure is kJ mol-1 Å-3. Values for the surface tension of water in these models vary significantly depending on the model used (e.g., between 0.0004 and 0.146 kJ mol-1 Å-3). [2] [3]

Electrostatic potential

The preferred unit of electrostatic potential is \(k_B \, T \, e_c^{-1}\) or \(R \, T \, e_c^{-1}\). If \(T \approx 298 \, \text{K}\), then \(k_B \, T \, e_c^{-1} = R \, T \, e_c^{-1} \approx 0.0256 \, \text{J} \, \text{C}^{-1} = 25.6 \, \text{mV}\).

[1]Sharp KA, Nicholls A, Fine RF, Honig B. Reconciling the magnitude of the microscopic and macroscopic hydrophobic effects. Science, 252, 106-109, 1991. DOI:10.1126/science.2011744.
[2](1, 2) Thomas DG, Chun J, Zhen C, Wei GW, Baker NA. Parameterization of a geometric flow implicit solvation model. J Comput Chem, 34, 687-695, 2013. DOI:10.1002/jcc.23181.
[3](1, 2) Wagoner JA and Baker NA. Assessing implicit models for nonpolar mean solvation forces: The importance of dispersion and volume terms. Proc Natl Acad Sci USA, 103, 8331-9336, 2006. DOI:10.1073/pnas.0600118103.