# Thermodynamics

Available since Sierra v1.5.0.

The ThermodynamicsInput can be used to straightforwardly calculate gas-phase thermodynamic quantities.

The results of a ThermodynamicsInput calculation are stored in the ThermodynamicsResult object.

## Example

The following examples demonstrate how to perform a thermodynamics calculation:

import sierra
from sierra.inputs import *

water = Molecule(pubchem="water")

# Calculate thermodynamic properties for water at GFN1-xTB level of theory
thermo_inp = ThermodynamicsInput(molecule=water, method=XTBMethod(model="GFN1"))
thermo_result = sierra.run(thermo_inp)
print("Thermodynamic properties for water:")
#> Thermodynamic properties for water:
print(thermo_result.gibbs_free_energy_total)
#> -5.767088376878814
print(thermo_result.heat_capacity)
#> 9.620470385781352e-06


## ThermodynamicsInput

#### Fields

All of the fields in SingleInput and the following:

temperature

The temperature for which thermodynamic quantities are calculated.

• Type: TemperatureQuantity
• Default: 300 K
pressure

The pressure for which thermodynamic quantities are calculated.

• Type: PressureQuantity
• Default: 1 atm

## ThermodynamicsResult

#### Fields

All of the fields in SingleResult and the following. For each field labeled *_correction, there is a corresponding field *_total. E.g: internal_energy_correction and internal_energy_total. In each case, the *_total value is the sum of the correction and the reference_energy.

reference_energy

The energy on top of which thermodynamic contributions of the energy are added. Usually, this is the electronic energy.

• Type: EnergyQuantity
internal_energy_correction

The internal energy of the system, relative to the reference_energy. The internal_energy_total can be obtained as U = reference_energy + internal_energy_correction.

• Type: EnergyQuantity
internal_energy_total

The total internal energy of the system, U = reference_energy + internal_energy_correction.

• Type: EnergyQuantity
enthalpy_correction

The enthalpy (H) of the system, relative to the reference_energy.

• Type: EnergyQuantity
enthalpy_total

The total enthalpy (H) of the system, H = reference_energy + enthalpy_correction.

• Type: EnergyQuantity
entropy

The total entropy (S) of the system.

• Type: EntropyQuantity
gibbs_free_energy_correction

The Gibbs free energy, G = H - TS, relative to the reference_energy.

• Type: EnergyQuantity
gibbs_free_energy_total

The total Gibbs free energy, G = reference_energy + gibbs_free_energy_correction.

• Type: EnergyQuantity
zero_point_energy_correction

The zero-point vibrational energy of the system, i.e. the energy due to molecular motion at zero Kelvin, relative to the reference_energy.

• Type: EnergyQuantity
zero_point_energy_total

For consistency, this value is defined as reference_energy + zero_point_energy_correction.

• Type: EnergyQuantity
heat_capacity

The heat capacity at constant pressure.

• Type: EntropyQuantity