Journal of Chemical Physics
Feedback | Help | Exit
The Journal of Chemical Physics
Search:
   
 
 
 
Previous Article
Chemical structure effects on the equilibrium and under shear properties of thin films in confined geometries: A molecular dynamics simulation study
We have made a comparative study of confined thin fluid films, composed of either n-decane or 4-propyl-heptane. The films are studied in equilibrium and under shear using molecular dynamics (MD) simul...
Next Article
Frequency spectra of two-band fluids and fluid mixtures: Mean spherical approximation and beyond
In the framework of a recently proposed approximation, we investigate here the frequency spectra of two-band fluids (fluids composed of particles with two independent Drude oscillators embedded) as we...

On the application of instantaneous normal mode analysis to long time dynamics of liquids

J. Chem. Phys. 103, 2169 (1995); doi:10.1063/1.469693

Issue Date: 8 August 1995

You are not logged in to this journal. Log in

G. V. Vijayadamodar and Abraham Nitzan
School of Chemistry, Tel Aviv University, Tel Aviv, 69978 Israel
While the applicability of instantaneous normal mode (INM) analysis of liquids to short time dynamics is in principle obvious, its relevance to long time dynamics is not clear. Recent attempts by Keyes and co-workers to apply information obtained from this analysis to self-diffusion in supercooled liquid argon is critically analyzed. By extending the range of frequencies studied we show that both imaginary and real branches of the density of modes are represented better, for large omega, by ln[rho(omega)]~omega2/T than by ln[rho(omega)]~omega4/T2 as advocated by Keyes [J. Chem. Phys. 101, 5081 (1994)]. However, since in the relevant frequency range the two fits almost overlap, the numerical results obtained by Keyes, showing good agreement with the simulation results for self-diffusion in supercooled liquid argon, remain valid even though implications for the frequency dependence of the barrier height distribution change. We also explore other possibilities for extracting information from the INM analysis: (1) The density of ``zero force modes,'' defined as the distribution of normal modes found at the bottom or top of their parabolic potential surfaces, can be computed with no appreciable additional numerical effort. This distribution provides a better representation than the total density of modes for the normal mode distribution at well bottoms and at saddles, however, we find that it makes little difference in quantitative analysis. (2) We suggest that the ratio rhou(omega)/rhos(omega) between the density of modes in the unstable and stable branches provide an estimate for the averaged barrier height distribution for large omega. Using this estimate in a transition state theory calculation of the average hopping time between locally stable liquid configurations and using the resulting time in a calculation of the self-diffusion coefficient yields a very good agreement with results of numerical simulation. ©1995 American Institute of Physics.
History: Received 14 February 1995; accepted 3 May 1995
Permalink: http://link.aip.org/link/?JCPSA6/103/2169/1
BUY THIS ARTICLE   (US$24)
Download PDF (275 kB) View Cart

KEYWORDS and PACS

Keywords
PACS
  • 61.20.Ne
    Structure of solids and liquids; crystallography Structure of liquids Structure of simple liquids
  • YEAR: 1995

RELATED DATABASES


To view database links for this article,
you need to log in.
To view database links for this article,
you need to log in.

PUBLICATION DATA

ISSN:
0021-9606 (print)   1089-7690 (online)
Publisher:
AIP is a member of CrossRef AIP

REFERENCES (31)
View in Separate Window/Tab
For access to fully linked references, you need to log in. For access to fully linked references, you need to Log in.
  1. Robert Zwanzig, J. Chem. Phys. 79, 4507 (1983).
  2. G. Seeley and T. Keyes, J. Chem. Phys. 91, 5581 (1989).
  3. B. Madan, T. Keyes, and G. Seeley, J. Chem. Phys. 92, 7565 (1990).
  4. B. Madan, T. Keyes, and G. Seeley, J. Chem. Phys. 94, 6762 (1991).
  5. G. Seeley, T. Keyes, and B. Madan, J. Phys. Chem. 96, 4047 (1992).
  6. B. Madan and T. Keyes, J. Chem. Phys. 98, 3342 (1993).
  7. P. Moore and T. Keyes, J. Chem. Phys. 100, 6709 (1994).
  8. T. Keyes, J. Chem. Phys. 101, 5081 (1994).
  9. I. Ohmine and H. Tanaka, Chem. Rev. 93, 2545 (1993).
  10. J. D. Bauer and D. F. Calef, Chem. Phys. Lett. 187, 391 (1991).
  11. B.-C. Xu and R. M. Stratt, J. Chem. Phys. 91, 5613 (1989).
  12. B.-C. Xu and R. M. Stratt, J. Chem. Phys. 92, 1923 (1990).
  13. K. Ganguly and R. M. Stratt, J. Chem. Phys. 97, 1980 (1992).
  14. Z. Chen and R. M. Stratt, J. Chem. Phys. 97, 5687 (1992).
  15. Z. Chen and R. M. Stratt, J. Chem. Phys. 97, 5696 (1992).
  16. M. Buchner, B. M. Ladanyi, and R. M. Stratt, J. Chem. Phys. 97, 8522 (1992).
  17. R. M. Stratt and M. Cho, J. Chem. Phys. 100, 6700 (1994).
  18. M. Cho, G. R. Flemming, S. Saito, I. Ohmine, and R. M. Stratt, J. Chem. Phys. 100, 6672 (1994).
  19. T.-M. Wu and R. F. Loring, J. Chem. Phys. 97, 8568 (1992).
  20. T.-M. Wu and R. F. Loring, J. Chem. Phys. 99, 8936 (1993).
  21. H. Stassen and Z. E. Gburski, Chem. Phys. Lett. 217, 325 (1994).
  22. B. Bagchi, J. Chem. Phys. 101, 9946 (1994).
  23. F. Stillinger and F. Weber, Phys. Rev. A 28, 2408 (1983);
    24. J. Chem. Phys. 80, 4434 (1984).
  24. Note that the definition of QB/Qm here is different from upsilonB/upsilonm, the ratio of phase space volumes in the barrier and well regions of Ref 8, by a factor of (in the harmonic approximation) omegaB/omegam where omegaB is the barrier frequency.
  25. For a pair of neighboring barrier and well associated with the same frequency omega and located at a fixed distance from each other, the vertical energy difference scales as omega2. This is expected to hold for modes of strong local character, i.e., large omega0.
  26. Numerically we see a large accumulation of k~0 modes in the rho(k) spectrum [see Fig. 4(a)], pointing to the importance of contributions not from saddles and not from well bottoms.
  27. We believe that the large estimate for the parameter alpha3z~0.43 (alpha is the number of atoms in the independent rearranging regions) results from this assumption. For a modest z = 6 this would imply alpha = 8. This implies that the dimension of the saddle—the number of downward directions and z is the typical saddle has 8 reactive directions, while the fit to the diffusion constant uses one such direction.
  28. Ensembles of 2000 configurations and of 50 configurations were used for the 100 particle system and the 400 particle system, respectively.
  29. This is done by defining k = x3/4 and using windows of constant width in x. The natural alternative, to use windows of fixed width in omega, i.e., deltak = 2k1/2deltaomega, with deltaomega = 0.1 was used to construct the histograms for rho(omega) and rho0(omega) in Figs. 1 and 2. Making the same choice here yields very similar results with somewhat bigger scatter of data in the large |k| range.
  30. A possible exception is lnrho0s(|k|) in the high density fluid, which we found to be represented almost as well by both forms, i.e., the best fit would be to exp(−alphakx) with 1<x<2.
  31. U. Mohanty, Phys. Rev. A 32, 3055 (1985).

CITING ARTICLES
For access to citing articles, you need to log in.
For access to citing articles, you need to Log in.


Copyright © American Institute of Physics