The following shows how to omit title information for @articles.
While for @articles omitting the title can be justified, for @books it would by simply wrong (try to find "Knuth, Donald. Addison-Wesley: Reading, Mass., 1986" not knowing the title is "TeX: The Program" ["Knuth 1986" could well be "The METAFONTbook"]).
Though in principle I would not advise doing this*, it can be done, so here you go.
Locate plain.bst on your machine, use kpsewhich plain.bst if you cannot find it.
Copy plain.bst to a location LaTeX can find it, the folder where your current .tex document resides is not too bad an option if you do not know where to put it.
Rename the file to, say plainnt.bst (as in "plain: no title" - not to be confused with nmatbib's plainnat.bst).
Open plainnt.bst and navigate to FUNCTION {article}, you will find the following lines
format.authors "author" output.check
new.block
format.title "title" output.check
new.block
just delete the title part, so the whole FUNCTION {article} reads:
FUNCTION {article}
{ output.bibitem
format.authors "author" output.check
new.block
crossref missing$
{ journal emphasize "journal" output.check
format.vol.num.pages output
format.date "year" output.check
}
{ format.article.crossref output.nonnull
format.pages output
}
if$
new.block
note output
fin.entry
}
In your document you now use \bibliographystyle{plainnt} instead of \bibliographystyle{plain}.
The MWE with plainnt.bst as above
\documentclass{article}
\begin{filecontents}{\jobname.bib}
@article{lawless2003,
author = {H. J. C. Berendsen and J. P. M. Postma and W. F. van Gunsteren and A. DiNola and J. R. Haak},
title = {Molecular dynamics with coupling to an external bath},
journal = {The Journal of Chemical Physics},
year = {1984},
volume = {81},
number = {8},
pages = {3685-3690},
url = {http://scitation.aip.org/content/aip/journal/jcp/81/8/10.1063/1.448118},
doi = {10.1063/1.448118},
}
\end{filecontents}
\begin{document}
\nocite{*}
\bibliography{\jobname}
\bibliographystyle{plainnt}
\end{document}
yields

*) Why not including the title is a not a very good idea.
First of all, there are quite a lot of bibliography style guides out there that require the title be included in the bibliography. I have seen (or at least I seem to remember to have) some journal citation styles that omit title information for articles.
My main points though are
- The reader will have to do more research to find the article: While normally finding the article is as easy as typing (or copy-and-pasting) the title into any search engine you like, here the reader will have to have the printed journal in front of them or will have to navigate to the journal website and leaf through the webpage to find the article.
- The title can give the reader a rough idea of what the article you just cited is about