Strange () appearing when using entry @misc in bibliography (revtex4.1)

For some reasons, natbib defines \natexlab to do nothing with its argument; such bit is added for distinguishing between references with the same author (in this case, none).

Fix: define \natexlab as revtex4-1 prefers.

\begin{filecontents*}{\jobname.bib}
@misc{lammps,
  title = {{LAMMPS website}},
  howpublished = {\url{http://lammps.sandia.gov}},
  note = {[Accessed: 10-October-2017]},
}

@misc{lammps_manual_fix_langevin,
  title = {{LAMMPS manual - fix langevin command}},
  howpublished = {\url{http://lammps.sandia.gov/doc/fix_langevin.html}},
  note = {[Accessed: 09-November-2017]},
}
\end{filecontents*}

\documentclass[aps,jmp,amsmath,amssymb,reprint]{revtex4-1}

\AtBeginDocument{\renewcommand{\natexlab}[1]{#1}}% <--- the fix

\begin{document}
Bla bla bla \cite{lammps} \cite{lammps_manual_fix_langevin}

\bibliography{\jobname}

\end{document}

Don't have the reputation to post as comment to the above.

Stumbled upon this post with a similar problem (in my case a leading .). I discovered that by changing the .bib file keys (i.e. lammps, lammps_manual_fix_langevin in your case) such that they differed in their first letter (i.e. a_lammps, lammps_manual_fix_langevin) resulted in complete removal.

\begin{filecontents*}{\jobname.bib}
  @misc{a_lammps,
    title = {{LAMMPS website}},
    howpublished = {\url{http://lammps.sandia.gov}},
    note = {[Accessed: 10-October-2017]},
  }

  @misc{lammps_manual_fix_langevin,
    title = {{LAMMPS manual - fix langevin command}},
    howpublished = {\url{http://lammps.sandia.gov/doc/fix_langevin.html}},
    note = {[Accessed: 09-November-2017]},
  }
\end{filecontents*}

\documentclass[aps,jmp,amsmath,amssymb,reprint]{revtex4-1}

\begin{document}

Bla bla bla \cite{a_lammps} \cite{lammps_manual_fix_langevin}

\bibliography{\jobname}

\end{document}