# Quasicontinuum Frequently Asked Questions (FAQ)

## General

1. How do I cite QC in publications?
The main citations for the QC method are:
• E. B. Tadmor, M. Ortiz and R. Phillips, "Quasicontinuum analysis of defects in solids", Phil. Mag. A, 73, 1529-1563 (1996).
• V. B. Shenoy, R. Miller, E. B. Tadmor, D. Rodney, R. Phillips and M. Ortiz, "An adaptive finite element approach to atomic-scale mechanics the quasicontinuum method", J. Mech. Phys. Solids, 47, 611-642 (1999).
To cite the QC code downloaded from this website, use (after replacing the "X"s):
• E. B. Tadmor and R. E. Miller, Quasicontinuum Code version X.X.
To cite the MultiBench code downloaded from this website, use (after replacing the "X"s):
• R. E. Miller and E. B. Tadmor, "A unified framework and performance benchmark of fourteen multiscale atomistic/continuum coupling methods", Model. Simul. Mater. Sci. Eng., 17, 053001 (2009).
• E. B. Tadmor and R. E. Miller, MultiBench Code version X.X.

2. Does the code do 3D simulations?
Yes and no. There is no fully 3D version of the code available for download. In the available code, the crystal structure is 3D, but it has a minimal periodic length in the z-direction so that deformation is limited to 2D.

Displacements can be in all three directions, but the periodicity out-of-plane means that the deformations can only vary in-plane. The resulting deformation is neither plane stress nor plane strain. Specifically we have
$u_1=u_1(X_1,X_2)$
$u_2=u_2(X_1,X_2)$
$u_3=u_3(X_1,X_2)$
so that the deformation gradient has certain components that are constrained:
$F_{13}=F_{23}=0$
and
$F_{33}=1$

3. I want to study my favorite material system, X. Can the QC code be used?
The QC method can, in priniciple, be used to study any polycrystalline material. However, there are two limiting factors in the actual code that you can download.

The first limitation is that the downloadable code is limited to simple crystals. This means crystals that can be described as a single atom attached to each Bravais lattice site (fcc and bcc are the most common examples). It cannot correctly treat structures like diamond cubic or hcp because the simplified implementation of the Cauchy-Born rule that was used does not allow for sub-lattice shuffling.

The second limitation is the availability of suitable interatomic models. The available code can use any EAM potential that is in the LAMMPS-compatible setfl or funcfl format (see the LAMMPS documentation for more details).

Bundled with the QC code, we provide some representative EAM potentials for the pure metals Ag, Al, Au, Cu, Mo, Ni, Pd and Pt. We also provide an EAM potential for the alloy system Al-Cu-Ag and an MGPT potential for Mo. For a ream of other EAM potentials, visit the NIST repository.

4. Does the QC method work at finite temperature (i.e. is it a dynamic method)?
The code available for download from this site is the static, zero Kelvin version of QC. It simulates quasi-static loading at zero temperature and uses energy minimization to find equilibrium configurations.

Due to a rather unfortunate persistence of some legacy code features in QC (which was originally built from an already heavily modified, 90s-era version of FEAPpv!), the variable called time is used to advance the loading. The has nothing to do with the "time" that we all know and love as a consequence of the second law of thermodynamics.

The finite temperature code described in

• Finite-temperature quasicontinuum: Molecular dynamics without all the atoms
L. M. Dupuy, E. B. Tadmor, R. E. Miller and R. Phillips
Physical Review Letters, 95, Art. No. 060202 (2005).
Abstract HTML Text
is still a research code and is not currently available for download.
5. Is the QC method a dynamic method (i.e. does it work at finite temperature)?
See the answer to the previous question.

## Support

1. I've run into a problem using the QC (or Multibench) code. What should I do?
Look for an answer in the documentation (tutorial and reference guide). If that doesn't help post a question on the QC forum (Community/Forum). See also the Contact page for more detailed instructions.
2. I found a bug in QC or MultiBench. What should I do?
Fill in a bug report form on the "Support/Bug Reports" page.
3. Are there any courses offered on QC that I can participate in?
Tadmor and Miller will be offering short courses on multiscale materials modeling which will include an introduction to QC. These course are based on their book "Modeling Materials: Continuum, Atomistic and Multiscale Techniques" (Cambridge University Press, 2011). For more information on the book and short courses, visit http://modelingmaterials.org.
4. I plotted the functions from the potential file X_somebody.fcn and they don't look like the functions in the paper, "An EAM potential for X" by Dr. Somebody. Why?
There are several possibilities here.

First, note that the pair potential is a function of distance, r, and usually it goes to infinity when r goes to zero. Therefore, the pair potential is stored as the function multiplied by r instead of just the function itself. To see just the pair potential, you must first divide by r.

Second, there are invariant transformations that can be applied to EAM potentials so as to rather dramatically change the form of the functions. However, so long as the other functions are changed in a corresponding way, there is no effect on the overall energy description. For more details on this, see Exercise 5.6 in the book "Modeling Materials" by Tadmor and Miller.

Finally, there may just be multiple variants of the potential floating around. The potentials provided with the QC code are believed to be accurate and correctly attributed, but people often change their potentials over time. See the NIST repository or openKIM for reliable sources of interatomic potentials with known provenance.