Virial stress

Virial stress is a measure of mechanical stress on an atomic scale for homogeneous systems. The name is derived from the Latin word vis, meaning force: "Virial is then derived from Latin as well, stemming from the word virias (plural of vis) meaning forces." ^[1] The expression of the (local) virial stress can be derived as the functional derivative of the free energy of a molecular system with respect to the deformation tensor. ^[2]

Volume averaged Definition

The instantaneous volume averaged virial stress is given by

${\displaystyle \tau _{ij}={\frac {1}{\Omega }}\sum _{k\in \Omega }\left(-m^{(k)}(u_{i}^{(k)}-{\bar {u}}_{i})(u_{j}^{(k)}-{\bar {u}}_{j})+{\frac {1}{2}}\sum _{\ell \in \Omega }(x_{i}^{(\ell )}-x_{i}^{(k)})f_{j}^{(k\ell )}\right)}$

where

• ${\displaystyle k}$ and ${\displaystyle \ell }$ are atoms in the domain,
• ${\displaystyle \Omega }$ is the volume of the domain,
• ${\displaystyle m^{(k)}}$ is the mass of atom k,
• ${\displaystyle u_{i}^{(k)}}$ is the i^th component of the velocity of atom k,
• ${\displaystyle {\bar {u}}_{j}}$ is the j^th component of the average velocity of atoms in the volume,
• ${\displaystyle x_{i}^{(k)}}$ is the i^th component of the position of atom k, and
• ${\displaystyle f_{i}^{(k\ell )}}$ is the i^th component of the force applied on atom ${\displaystyle k}$ by atom ${\displaystyle \ell }$.

At zero kelvin, all velocities are zero so we have

${\displaystyle \tau _{ij}={\frac {1}{2\Omega }}\sum _{k,\ell \in \Omega }(x_{i}^{(\ell )}-x_{i}^{(k)})f_{j}^{(k\ell )}}$.

This can be thought of as follows. The τ₁₁ component of stress is the force in the x₁-direction divided by the area of a plane perpendicular to that direction. Consider two adjacent volumes separated by such a plane. The 11-component of stress on that interface is the sum of all pairwise forces between atoms on the two sides.

The volume averaged virial stress is then the ensemble average of the instantaneous volume averaged virial stress.

In a three dimensional, isotropic system, at equilibrium the "instantaneous" atomic pressure is usually defined as the average over the diagonals of the negative stress tensor:

${\displaystyle {\mathcal {P}}_{at}=-{\frac {1}{3}}Tr(\tau ).}$

The pressure then is the ensemble average of the instantaneous pressure ^[3]

${\displaystyle P_{at}=\langle {\mathcal {P}}_{at}\rangle .}$

This pressure is the average pressure in the volume ${\displaystyle \Omega }$.

Equivalent Definition

It's worth noting that some articles and textbook ^[3] use a slightly different but equivalent version of the equation

${\displaystyle \tau _{ij}={\frac {1}{\Omega }}\sum _{k\in \Omega }\left(-m^{(k)}(u_{i}^{(k)}-{\bar {u}}_{i})(u_{j}^{(k)}-{\bar {u}}_{j})-{\frac {1}{2}}\sum _{\ell \in \Omega }x_{i}^{(k\ell )}f_{j}^{(k\ell )}\right)}$

where ${\displaystyle x_{i}^{(k\ell )}}$ is the i^th component of the vector oriented from the ${\displaystyle \ell }$^th atoms to the k^th calculated via the difference

${\displaystyle x_{i}^{k\ell }=x_{i}^{(k)}-x_{i}^{(\ell )}}$

Both equation being strictly equivalent, the definition of the vector can still lead to confusion.

Derivation

The virial pressure can be derived, using the virial theorem and splitting forces between particles and the container ^[4] or, alternatively, via direct application of the defining equation ${\displaystyle P=-{\frac {\partial F(N,V,T)}{\partial V}}}$ and using scaled coordinates in the calculation.

Inhomogeneous Systems

If the system is not homogeneous in a given volume the above (volume averaged) pressure is not a good measure for the pressure. In inhomogeneous systems the pressure depends on the position and orientation of the surface on which the pressure acts. Therefore, in inhomogeneous systems a definition of a local pressure is needed. ^[5] As a general example for a system with inhomogeneous pressure you can think of the pressure in the atmosphere of the earth which varies with height.

Instantaneous local virial stress

The (local) instantaneous virial stress is given by: ^[2]

${\displaystyle \tau _{ab}({\vec {r}})=-\sum _{i=1}^{N}\delta ({\vec {r}}-{\vec {r}}^{(i)})\left(m^{(i)}u_{a}^{(i)}u_{b}^{(i)}+{\frac {1}{2}}\sum _{j=1,j\neq i}^{N}({\vec {r}}^{(i)}-{\vec {r}}^{(j)})_{a}{\vec {f}}_{b}^{(ij)}\right),}$

Measuring the virial pressure in molecular simulations

The virial pressure can be measured via the formulas above or using volume rescaling trial moves. ^[6]

See also

• Virial theorem

References

1. Wasalwar, Yash (May 23, 2023). "Spotlight: the Virial Theorem". Medium. Archived from the original on February 3, 2024.
2. Morante, S., G. C. Rossi, and M. Testa. "The stress tensor of a molecular system: An exercise in statistical mechanics." The Journal of chemical physics 125.3 (2006): 034101, http://aip.scitation.org/doi/abs/10.1063/1.2214719.
3. Allen, MP; Tildesley, DJ (1991). Clarendon Press (ed.). Computer Simulations of Liquids. Oxford. pp. 46–50.
4. Navet, M.; Jamin, E.; Feix, M. R. (1980-02-01). "" Virial " pressure of the classical one-component plasma". Journal de Physique Lettres. 41 (3): 69–73. doi:10.1051/jphyslet:0198000410306900. ISSN   0302-072X. S2CID   122678419.
5. Loison, Claire (2005). Numerical Simulations of a Smectic Lamellar Phase of Amphiphilic Molecules. Cuvillier Verlag. ISBN   978-3-86537-655-8.
6. Miguel, Enrique de; Jackson, George (2006-10-30). "The nature of the calculation of the pressure in molecular simulations of continuous models from volume perturbations". The Journal of Chemical Physics. 125 (16): 164109. Bibcode:2006JChPh.125p4109D. doi:10.1063/1.2363381. hdl: 10272/9584 . ISSN   0021-9606. PMID   17092065.

External links

• Physical Interpretation of the volume averaged Virial Stress
• Allen, M.P.; Tildesley, DJ. Computer Simulation of Liquids (PDF). Archived from the original (PDF) on 2016-12-21.
• 2017 edition (second): Allen, Michael Patrick; Tildesley, Dominic J. (2017). Computer simulation of liquids (PDF) (2nd ed.). Oxford: Oxford University Press. ISBN   9780198803201. Archived (PDF) from the original on November 26, 2022.
• Python and Fortran code examples for Computer Simulation of Liquids
• Zhou, Min (8 September 2003). "A new look at the atomic level virial stress: on continuum-molecular system equivalence" (PDF). Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences. 459 (2037): 2347–2392. doi:10.1098/rspa.2003.1127. ISSN   1364-5021. Archived (PDF) from the original on February 3, 2024.