Determination of anharmonic free energy contributions: Low temperature phases of the LennardJones system
We investigate a general method to calculate the free energy of crystalline solids by considering the harmonic approximation and quasistatically switching the anharmonic contribution. The advantage of this method is that the harmonic approximation provides an already very accurate estimate of the free energy, and therefore the anharmonic term is numerically very small and can be determined to high accuracy. We further show that the anharmonic contribution to the free energy satisfies a number of exact inequalities that place constraints on its magnitude and allows approximate but fast and accurate estimates. The method is implemented into a readily available general software by combining the code HOODLT (Highly Optimized Object Oriented Dynamic Lattice Theory) for the harmonic part and the molecular dynamics (MD) simulation package HOOMDblue for the anharmonic part. We use the method to calculate the low temperature phase diagram for LennardJones particles. We demonstrate that hcp is the equilibrium phase at low temperature and pressure and obtain the coexistence curve with the fcc phase, which exhibits reentrant behavior. Furthermore, several implications of the method are discussed.
 Authors:

^{[1]};
^{[2]};
^{[2]}
 Boston Univ., Boston, MA (United States); Univ. de Barcelona, Barcelona (Spain)
 Ames Lab. and Iowa State Univ., Ames, IA (United States)
 Publication Date:
 Report Number(s):
 ISJ8994
Journal ID: ISSN 00219606; JCPSA6
 Grant/Contract Number:
 AC0207CH11358
 Type:
 Accepted Manuscript
 Journal Name:
 Journal of Chemical Physics
 Additional Journal Information:
 Journal Volume: 144; Journal Issue: 12; Journal ID: ISSN 00219606
 Publisher:
 American Institute of Physics (AIP)
 Research Org:
 Ames Laboratory (AMES), Ames, IA (United States)
 Sponsoring Org:
 USDOE
 Country of Publication:
 United States
 Language:
 English
 Subject:
 36 MATERIALS SCIENCE; free energy; phase coexistence; LennardJones potential; inequalities; phase diagram; lattice theory; computer software
 OSTI Identifier:
 1254299
 Alternate Identifier(s):
 OSTI ID: 1242866
Calero, C., Knorowski, C., and Travesset, A.. Determination of anharmonic free energy contributions: Low temperature phases of the LennardJones system. United States: N. p.,
Web. doi:10.1063/1.4944069.
Calero, C., Knorowski, C., & Travesset, A.. Determination of anharmonic free energy contributions: Low temperature phases of the LennardJones system. United States. doi:10.1063/1.4944069.
Calero, C., Knorowski, C., and Travesset, A.. 2016.
"Determination of anharmonic free energy contributions: Low temperature phases of the LennardJones system". United States.
doi:10.1063/1.4944069. https://www.osti.gov/servlets/purl/1254299.
@article{osti_1254299,
title = {Determination of anharmonic free energy contributions: Low temperature phases of the LennardJones system},
author = {Calero, C. and Knorowski, C. and Travesset, A.},
abstractNote = {We investigate a general method to calculate the free energy of crystalline solids by considering the harmonic approximation and quasistatically switching the anharmonic contribution. The advantage of this method is that the harmonic approximation provides an already very accurate estimate of the free energy, and therefore the anharmonic term is numerically very small and can be determined to high accuracy. We further show that the anharmonic contribution to the free energy satisfies a number of exact inequalities that place constraints on its magnitude and allows approximate but fast and accurate estimates. The method is implemented into a readily available general software by combining the code HOODLT (Highly Optimized Object Oriented Dynamic Lattice Theory) for the harmonic part and the molecular dynamics (MD) simulation package HOOMDblue for the anharmonic part. We use the method to calculate the low temperature phase diagram for LennardJones particles. We demonstrate that hcp is the equilibrium phase at low temperature and pressure and obtain the coexistence curve with the fcc phase, which exhibits reentrant behavior. Furthermore, several implications of the method are discussed.},
doi = {10.1063/1.4944069},
journal = {Journal of Chemical Physics},
number = 12,
volume = 144,
place = {United States},
year = {2016},
month = {3}
}