This paper generalizes isomorph theory to systems that are not in thermal equilibrium. The systems are assumed to be R-simple, i.e., to have a potential energy that as a function of all particle coordinates R obeys the hidden-scale-invariance condition U(Ra) < U(Rb) ⇒ U(λRa) < U(λRb). “Systemic isomorphs” are introduced as lines of constant excess entropy in the phase diagram defined by density and systemic temperature, which is the temperature of the equilibrium state point with the average potential energy equal to U(R). The dynamics is invariant along a systemic isomorph if there is a constant ratio between the systemic and the bath temperature. In thermal equilibrium, the systemic temperature is equal to the bath temperature and the original isomorph formalism is recovered. The new approach rationalizes within a consistent framework previously published observations of isomorph invariance in simulations involving nonlinear steady-state shear flows, zero-temperature plastic flows, and glass-state isomorphs. This paper relates briefly to granular media, physical aging, and active matter. Finally, we discuss the possibility that the energy unit defining the reduced quantities should be based on the systemic rather than the bath temperature.

1.
T. B.
Schrøder
and
J. C.
Dyre
, “
Simplicity of condensed matter at its core: Generic definition of a Roskilde-simple system
,”
J. Chem. Phys.
141
,
204502
(
2014
).
2.
N.
Gnan
,
T. B.
Schrøder
,
U. R.
Pedersen
,
N. P.
Bailey
, and
J. C.
Dyre
, “
Pressure-energy correlations in liquids. IV. “Isomorphs” in liquid phase diagrams
,”
J. Chem. Phys.
131
,
234504
(
2009
).
3.
J. C.
Dyre
, “
Perspective: Excess-entropy scaling
,”
J. Chem. Phys.
149
,
210901
(
2018
).
4.
N. P.
Bailey
,
U. R.
Pedersen
,
N.
Gnan
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Pressure-energy correlations in liquids. II. Analysis and consequences
,”
J. Chem. Phys.
129
,
184508
(
2008
).
5.
A. K.
Bacher
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
The EXP pair-potential system. II. Fluid phase isomorphs
,”
J. Chem. Phys.
149
,
114502
(
2018
).
6.
U. R.
Pedersen
,
N. P.
Bailey
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Strong pressure-energy correlations in van der Waals liquids
,”
Phys. Rev. Lett.
100
,
015701
(
2008
).
7.
N. P.
Bailey
,
U. R.
Pedersen
,
N.
Gnan
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Pressure-energy correlations in liquids. I. Results from computer simulations
,”
J. Chem. Phys.
129
,
184507
(
2008
).
8.
T. S.
Ingebrigtsen
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Isomorphs in model molecular liquids
,”
J. Phys. Chem. B
116
,
1018
1034
(
2012
).
9.
E. N. C.
Andrade
, “
The viscosity of liquids
,”
Nature
125
,
582
584
(
1930
).
10.
E. N. C.
Andrade
, “
A theory of the viscosity of liquids. Part I
,”
London, Edinburgh, Dublin Philos. Mag. J. Sci.
17
,
497
511
(
1934
).
11.
J. C.
Dyre
, “
Simple liquids’ quasiuniversality and the hard-sphere paradigm
,”
J. Phys.: Condens. Matter
28
,
323001
(
2016
).
12.
O.
Klein
, “
Om det osmotiska trycket hos en elektrolyt
,”
Medd. K. Vetenskapsakad. Nobelinst.
5
,
1
9
(
1919
).
13.
W. G.
Hoover
,
S. G.
Gray
, and
K. W.
Johnson
, “
Thermodynamic properties of the fluid and solid phases for inverse power potentials
,”
J. Chem. Phys.
55
,
1128
1136
(
1971
).
14.
Y.
Hiwatari
,
H.
Matsuda
,
T.
Ogawa
,
N.
Ogita
, and
A.
Ueda
, “
Molecular dynamics studies on the soft-core model
,”
Prog. Theor. Phys.
52
,
1105
1123
(
1974
).
15.
Y.
Rosenfeld
, “
Relation between the transport coefficients and the internal entropy of simple systems
,”
Phys. Rev. A
15
,
2545
2549
(
1977
).
16.
D. M.
Heyes
,
D.
Dini
, and
A. C.
Brańka
, “
Scaling of Lennard-Jones liquid elastic moduli, viscoelasticity and other properties along fluid-solid coexistence
,”
Phys. Status Solidi B
252
,
1514
1525
(
2015
).
17.
M. P.
Allen
and
D. J.
Tildesley
,
Computer Simulation of Liquids
(
Oxford Science Publications
,
1987
).
18.
D.
Frenkel
and
B.
Smit
,
Understanding Molecular Simulation
(
Academic Press
,
2002
).
19.
L. A.
Roed
,
D.
Gundermann
,
J. C.
Dyre
, and
K.
Niss
, “
Communication: Two measures of isochronal superposition
,”
J. Chem. Phys.
139
,
101101
(
2013
).
20.
W.
Xiao
,
J.
Tofteskov
,
T. V.
Christensen
,
J. C.
Dyre
, and
K.
Niss
, “
Isomorph theory prediction for the dielectric loss variation along an isochrone
,”
J. Non-Cryst. Solids
407
,
190
195
(
2015
).
21.
K.
Niss
, “
Mapping isobaric aging onto the equilibrium phase diagram
,”
Phys. Rev. Lett.
119
,
115703
(
2017
).
22.
H. W.
Hansen
,
A.
Sanz
,
K.
Adrjanowicz
,
B.
Frick
, and
K.
Niss
, “
Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers
,”
Nat. Commun.
9
,
518
(
2018
).
23.
T. B.
Schrøder
,
N.
Gnan
,
U. R.
Pedersen
,
N. P.
Bailey
, and
J. C.
Dyre
, “
Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems
,”
J. Chem. Phys.
134
,
164505
(
2011
).
24.
T.-J.
Yoon
,
M. Y.
Ha
,
E. A.
Lazar
,
W. B.
Lee
, and
Y.-W.
Lee
, “
Topological extension of the isomorph theory based on the Shannon entropy
,”
Phys. Rev. E
100
,
012118
(
2019
).
25.
A. K.
Bacher
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
The EXP pair-potential system. I. Fluid phase isotherms, isochores, and quasiuniversality
,”
J. Chem. Phys.
149
,
114501
(
2019
).
26.
A. K.
Bacher
,
U. R.
Pedersen
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
The EXP pair-potential system. IV. Isotherms, isochores, and isomorphs in the two crystalline phases
,”
J. Chem. Phys.
152
,
094505
(
2020
).
27.
D.
Fragiadakis
and
C. M.
Roland
, “
Intermolecular distance and density scaling of dynamics in molecular liquids
,”
J. Chem. Phys.
150
,
204501
(
2019
).
28.
K.
Koperwas
,
A.
Grzybowski
, and
M.
Paluch
, “
Virial-potential energy correlation and its relation to the density scaling for quasi-real model systems
,” arXiv:2004.04499 (
2020
).
29.
T. S.
Ingebrigtsen
and
H.
Tanaka
, “
Effect of size polydispersity on the nature of Lennard-Jones liquids
,”
J. Phys. Chem. B
119
,
11052
11062
(
2015
).
30.
D. E.
Albrechtsen
,
A. E.
Olsen
,
U. R.
Pedersen
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Isomorph invariance of the structure and dynamics of classical crystals
,”
Phys. Rev. B
90
,
094106
(
2014
).
31.
T. S.
Ingebrigtsen
,
J. R.
Errington
,
T. M.
Truskett
, and
J. C.
Dyre
, “
Predicting how nanoconfinement changes the relaxation time of a supercooled liquid
,”
Phys. Rev. Lett.
111
,
235901
(
2013
).
32.
A. A.
Veldhorst
,
J. C.
Dyre
, and
T. B.
Schrøder
, “
Scaling of the dynamics of flexible Lennard-Jones chains
,”
J. Chem. Phys.
141
,
054904
(
2014
).
33.
F.
Hummel
,
G.
Kresse
,
J. C.
Dyre
, and
U. R.
Pedersen
, “
Hidden scale invariance of metals
,”
Phys. Rev. B
92
,
174116
(
2015
).
34.
L.
Friedeheim
,
J. C.
Dyre
, and
N. P.
Bailey
, “
Hidden scale invariance at high pressures in gold and five other face-centered-cubic metal crystals
,”
Phys. Rev. E
99
,
022142
(
2019
).
35.
A. A.
Veldhorst
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Invariants in the Yukawa system’s thermodynamic phase diagram
,”
Phys. Plasmas
22
,
073705
(
2015
).
36.
P.
Tolias
and
F.
Lucco Castello
, “
Isomorph-based empirically modified hypernetted-chain approach for strongly coupled Yukawa one-component plasmas
,”
Phys. Plasmas
26
,
043703
(
2019
).
37.
J. C.
Dyre
, “
Hidden scale invariance in condensed matter
,”
J. Phys. Chem. B
118
,
10007
10024
(
2014
).
38.
C.
Alba-Simionesco
,
A.
Cailliaux
,
A.
Alegría
, and
G.
Tarjus
, “
Scaling out the density dependence of the alpha relaxation in glass-forming polymers
,”
Europhys. Lett.
68
,
58
64
(
2004
).
39.
C. M.
Roland
,
S.
Hensel-Bielowka
,
M.
Paluch
, and
R.
Casalini
, “
Supercooled dynamics of glass-forming liquids and polymers under hydrostatic pressure
,”
Rep. Prog. Phys.
68
,
1405
1478
(
2005
).
40.
T. B.
Schrøder
,
U. R.
Pedersen
,
N. P.
Bailey
,
S.
Toxvaerd
, and
J. C.
Dyre
, “
Hidden scale invariance in molecular van der Waals liquids: A simulation study
,”
Phys. Rev. E
80
,
041502
(
2009
).
41.
D.
Gundermann
,
U. R.
Pedersen
,
T.
Hecksher
,
N. P.
Bailey
,
B.
Jakobsen
,
T.
Christensen
,
N. B.
Olsen
,
T. B.
Schrøder
,
D.
Fragiadakis
,
R.
Casalini
,
C. M.
Roland
,
J. C.
Dyre
, and
K.
Niss
, “
Predicting the density–scaling exponent of a glass–forming liquid from Prigogine–Defay ratio measurements
,”
Nat. Phys.
7
,
816
821
(
2011
).
42.
C. M.
Roland
,
R.
Casalini
, and
M.
Paluch
, “
Isochronal temperature–pressure superpositioning of the alpha–relaxation in type-A glass formers
,”
Chem. Phys. Lett.
367
,
259
264
(
2003
).
43.
K. L.
Ngai
,
R.
Casalini
,
S.
Capaccioli
,
M.
Paluch
, and
C. M.
Roland
, “
Do theories of the glass transition, in which the structural relaxation time does not define the dispersion of the structural relaxation, need revision?
,”
J. Phys. Chem. B
109
,
17356
17360
(
2005
).
44.
A. I.
Nielsen
,
S.
Pawlus
,
M.
Paluch
, and
J. C.
Dyre
, “
Pressure dependence of the dielectric loss minimum slope for ten molecular liquids
,”
Philos. Mag.
88
,
4101
4108
(
2008
).
45.
A. K.
Bacher
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Explaining why simple liquids are quasi-universal
,”
Nat. Commun.
5
,
5424
(
2014
).
46.
L.
Costigliola
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Freezing and melting line invariants of the Lennard-Jones system
,”
Phys. Chem. Chem. Phys.
18
,
14678
14690
(
2016
).
47.
U. R.
Pedersen
,
L.
Costigliola
,
N. P.
Bailey
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Thermodynamics of freezing and melting
,”
Nat. Commun.
7
,
12386
(
2016
).
48.
I. H.
Bell
, “
Probing the link between residual entropy and viscosity of molecular fluids and model potentials
,”
Proc. Natl. Acad. Sci. U. S. A.
116
,
4070
4079
(
2019
).
49.
L.
Costigliola
,
D. M.
Heyes
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Revisiting the Stokes-Einstein relation without a hydrodynamic diameter
,”
J. Chem. Phys.
150
,
021101
(
2019
).
50.
L.
Separdar
,
N. P.
Bailey
,
T. B.
Schrøder
,
S.
Davatolhagh
, and
J. C.
Dyre
, “
Isomorph invariance of Couette shear flows simulated by the SLLOD equations of motion
,”
J. Chem. Phys.
138
,
154505
(
2013
).
51.
E.
Lerner
,
N. P.
Bailey
, and
J. C.
Dyre
, “
Density scaling and quasiuniversality of flow-event statistics for athermal plastic flows
,”
Phys. Rev. E
90
,
052304
(
2014
).
52.
Y.
Jiang
,
E. R.
Weeks
, and
N. P.
Bailey
, “
Isomorph invariance of dynamics of sheared glassy systems
,”
Phys. Rev. E
100
,
053005
(
2019
).
53.
T. S.
Ingebrigtsen
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
What is a simple liquid?
,”
Phys. Rev. X
2
,
011011
(
2012
).
54.
J. C.
Dyre
, “
Isomorph theory of physical aging
,”
J. Chem. Phys.
148
,
154502
(
2018
).
55.
S.
Chandrasekhar
, “
Stochastic problems in physics and astronomy
,”
Rev. Mod. Phys.
15
,
1
89
(
1943
).
56.
L. E.
Reichl
,
A Modern Course in Statistical Physics
, 4th ed. (
Wiley VCH
,
2016
).
57.
B. D.
Todd
and
P. J.
Daivis
, “
Homogeneous non-equilibrium molecular dynamics simulations of viscous flow: Techniques and applications
,”
Mol. Simul.
33
,
189
229
(
2007
).
58.
A.
Puglisi
,
A.
Sarracino
, and
A.
Vulpiani
, “
Temperature in and out of equilibrium: A review of concepts, tools and attempts
,”
Phys. Rep.
709-710
,
1
60
(
2017
).
59.
J. G.
Powles
,
G.
Rickayzen
, and
D. M.
Heyes
, “
Temperatures: Old, new and middle aged
,”
Mol. Phys.
103
,
1361
1373
(
2005
).
60.
A.
Barrat
,
J.
Kurchan
,
V.
Loreto
, and
M.
Sellitto
, “
Edwards’ measures for powders and glasses
,”
Phys. Rev. Lett.
85
,
5034
5037
(
2000
).
61.
L.
Leuzzi
, “
A stroll among effective temperatures in aging systems: Limits and perspectives
,”
J. Non-Cryst. Solids
355
,
686
693
(
2009
).
62.
L. F.
Cugliandolo
, “
The effective temperature
,”
J. Phys. A: Math. Theor.
44
,
483001
(
2011
).
63.
L.
Berthier
,
J.-L.
Barrat
, and
J.
Kurchan
, “
A two-time-scale, two-temperature scenario for nonlinear rheology
,”
Phys. Rev. E
61
,
5464
5472
(
2000
).
64.
G. W.
Scherer
,
Relaxation in Glass and Composites
(
Wiley
,
New York
,
1986
).
65.
T. S.
Ingebrigtsen
,
L.
Bøhling
,
T. B.
Schrøder
, and
J. C.
Dyre
, “
Thermodynamics of condensed matter with strong pressure-energy correlations
,”
J. Chem. Phys.
136
,
061102
(
2012
).
66.
L.
Bøhling
,
T. S.
Ingebrigtsen
,
A.
Grzybowski
,
M.
Paluch
,
J. C.
Dyre
, and
T. B.
Schrøder
, “
Scaling of viscous dynamics in simple liquids: Theory, simulation and experiment
,”
New J. Phys.
14
,
113035
(
2012
).
67.
J. C.
Dyre
, “
Isomorphs, hidden scale invariance, and quasiuniversality
,”
Phys. Rev. E
88
,
042139
(
2013
).
68.
W.
Kob
and
H. C.
Andersen
, “
Testing mode-coupling theory for a supercooled binary Lennard-Jones mixture I: The van Hove correlation function
,”
Phys. Rev. E
51
,
4626
4641
(
1995
).
69.
D. J.
Evans
and
G. P.
Morriss
, “
Nonlinear-response theory for steady planar Couette flow
,”
Phys. Rev. A
30
,
1528
1530
(
1984
).
70.
F.
Radjai
,
J.-N.
Roux
, and
A.
Daouadji
, “
Modeling granular materials: Century-long research across scales
,”
J. Eng. Mech.
143
,
04017002
(
2017
).
71.
A.
Baule
,
F.
Morone
,
H. J.
Herrmann
, and
H. A.
Makse
, “
Edwards statistical mechanics for jammed granular matter
,”
Rev. Mod. Phys.
90
,
015006
(
2018
).
72.
R. P.
Behringer
and
B.
Chakraborty
, “
The physics of jamming for granular materials: A review
,”
Rep. Prog. Phys.
82
,
012601
(
2019
).
73.
S. F.
Edwards
and
R. B. S.
Oakeshott
, “
Theory of powders
,”
Physica A
157
,
1080
1090
(
1989
).
74.
A.
Mehta
and
S. F.
Edwards
, “
Statistical mechanics of powder mixtures
,”
Physica A
157
,
1091
1100
(
1989
).
75.
J. C.
Dyre
, “
The glass transition and elastic models of glass-forming liquids
,”
Rev. Mod. Phys.
78
,
953
972
(
2006
).
76.
F.
Simon
, “
Über den Zustand der unterkühlten Flüssigkeiten und Glässer
,”
Z. Anorg. Allg. Chem.
203
,
219
227
(
1931
).
77.
L. C. E.
Struik
,
Physical Aging in Amorphous Polymers and Other Materials
(
Elsevier
,
Amsterdam
,
1978
).
78.
I. M.
Hodge
, “
Physical aging in polymer glasses
,”
Science
267
,
1945
1947
(
1995
).
79.
G. B.
McKenna
and
S. L.
Simon
, “
50th anniversary perspective: Challenges in the dynamics and kinetics of glass-forming polymers
,”
Macromolecules
50
,
6333
6361
(
2017
).
80.
B.
Ruta
,
E.
Pineda
, and
Z.
Evenson
, “
Relaxation processes and physical aging in metallic glasses
,”
J. Phys.: Condens. Matter
29
,
503002
(
2017
).
81.
T.
Hecksher
,
N. B.
Olsen
, and
J. C.
Dyre
, “
Fast contribution to the activation energy of a glass-forming liquid
,”
Proc. Natl. Acad. Sci. U. S. A.
116
,
16736
16741
(
2019
).
82.
M. E.
Cates
, “
Diffusive transport without detailed balance in motile bacteria: Does microbiology need statistical physics?
,”
Rep. Prog. Phys.
75
,
042601
(
2012
).
83.
M. C.
Marchetti
,
J. F.
Joanny
,
S.
Ramaswamy
,
T. B.
Liverpool
,
J.
Prost
,
M.
Rao
, and
R. A.
Simha
, “
Hydrodynamics of soft active matter
,”
Rev. Mod. Phys.
85
,
1143
1189
(
2013
).
84.
C.
Maggi
,
U. M. B.
Marconi
,
N.
Gnan
, and
R.
DiLeonardo
, “
Multidimensional stationary probability distribution for interacting active particles
,”
Sci. Rep.
5
,
10742
(
2015
).
85.
Y.
Fily
and
M. C.
Marchetti
, “
Athermal phase separation of self-propelled particles with no alignment
,”
Phys. Rev. Lett.
108
,
235702
(
2012
).
86.
D.
Loi
,
S.
Mossa
, and
L. F.
Cugliandolo
, “
Effective temperature of active matter
,”
Phys. Rev. E
77
,
051111
(
2008
).
You do not currently have access to this content.