dorsal/arxiv
View SchemaOn the calculation of diffusion coefficients in confined fluids and interfaces with an application to the liquid-vapor interface of water
| Authors | Pu Liu, Edward Harder, B. J. Berne |
|---|---|
| Categories | |
| ArXiv ID | physics/0311084 |
| URL | https://arxiv.org/abs/physics/0311084 |
Abstract
We propose a general methodology for calculating the self-diffusion tensor from molecular dynamics for a liquid with a liquid-gas or liquid-solid interface. The standard method used in bulk fluids, based on computing the mean square displacement as a function of time and extracting the asymptotic linear time dependence from this, is not valid for systems with interfaces or for confined fluids. The method proposed here is based on imposing virtual boundary conditions on the molecular system and computing survival probabilities and specified time correlation functions in different layers of the fluid up to and including the interfacial layer. By running dual simulations, one based on MD and the other based on Langevin dynamics, using the same boundary conditions, one can fit the Langevin survival probability at long times to the MD computed survival probability, thereby determining the diffusion coefficient as a function of distance of the layers from the interface. We compute the elements of the diffusion tensor of water as a function of distance from the liquid vapor interface of water. Far from the interface the diffusion tensor is found to be isotropic, as expected, and the diffusion coefficient has the value $D\approx$ .22\AA$^2$/psec in agreement with what is found in the bulk liquid. In the interfacial region the diffusion tensor is axially anisotropic, with values of $D_{\parallel}\approx$. 8\AA$^2$/psec and $D_{\perp}\approx$. 5\AA$^2$/psec for the components parallel and normal the interface surface respectively. We also show that diffusion in confined geometries can be calculated by imposing appropriate boundary conditions on the molecular system and computing time correlation functions of the eigenfunctions of the diffusion operator corresponding to the same boundary conditions.
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"date_created": "2026-03-02T18:00:46.299000Z",
"date_modified": "2026-03-02T18:00:46.299000Z",
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"abstract": "We propose a general methodology for calculating the self-diffusion tensor\nfrom molecular dynamics for a liquid with a liquid-gas or liquid-solid\ninterface. The standard method used in bulk fluids, based on computing the mean\nsquare displacement as a function of time and extracting the asymptotic linear\ntime dependence from this, is not valid for systems with interfaces or for\nconfined fluids. The method proposed here is based on imposing virtual boundary\nconditions on the molecular system and computing survival probabilities and\nspecified time correlation functions in different layers of the fluid up to and\nincluding the interfacial layer. By running dual simulations, one based on MD\nand the other based on Langevin dynamics, using the same boundary conditions,\none can fit the Langevin survival probability at long times to the MD computed\nsurvival probability, thereby determining the diffusion coefficient as a\nfunction of distance of the layers from the interface. We compute the elements\nof the diffusion tensor of water as a function of distance from the liquid\nvapor interface of water. Far from the interface the diffusion tensor is found\nto be isotropic, as expected, and the diffusion coefficient has the value\n$D\\approx$ .22\\AA$^2$/psec in agreement with what is found in the bulk liquid.\nIn the interfacial region the diffusion tensor is axially anisotropic, with\nvalues of $D_{\\parallel}\\approx$. 8\\AA$^2$/psec and $D_{\\perp}\\approx$.\n5\\AA$^2$/psec for the components parallel and normal the interface surface\nrespectively. We also show that diffusion in confined geometries can be\ncalculated by imposing appropriate boundary conditions on the molecular system\nand computing time correlation functions of the eigenfunctions of the diffusion\noperator corresponding to the same boundary conditions.",
"arxiv_id": "physics/0311084",
"authors": [
"Pu Liu",
"Edward Harder",
"B. J. Berne"
],
"categories": [
"physics.chem-ph"
],
"title": "On the calculation of diffusion coefficients in confined fluids and interfaces with an application to the liquid-vapor interface of water",
"url": "https://arxiv.org/abs/physics/0311084"
},
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"variant": "snapshot-2026-03-01",
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