The following are preferred APBS units.

# Numbers¶

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

# Length¶

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

# Volume¶

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).

# Temperature¶

The preferred unit of temperature is Kelvin (K).

# Charge¶

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

# Energy¶

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.  However, these values can vary significantly depending on the model used.  

# Pressure¶

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).  

# 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 $$k_B \, T \, e_c^{-1} = R \, T \, e_c^{-1} \approx 0.0256 \, \text{J} \, \text{C}^{-1} = 25.6 \, \text{mV}$$.

  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.
  (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.
  (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.