<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">I also had a similar question in my head for a while about the ubiquitous use of ab initio == quantum mechanical model in materials science and physics literature (nanomechanics as well, for what it's worth). Mostly because some of the techniques include things that are definitely approximations or involve some sort of parameterization. Now, compared to empirical potentials, DFT, Hartree-Fock and even many Tight Binding approaches are certainly 'ab initio' because of how and where the approximations are made... and this seems to be the justification for using 'ab initio' in this manner. Besides, it sounds cool when written or said... a reason whose influence probably shouldn't be underestimated.<div><div><br></div><div>Dave</div><div><br><div><div>On Dec 21, 2008, at 7:47 PM, Leonardo Matheus wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div class="gmail_quote">A small doubt that I had while following this discussion, please someone correct-me if I'm wrong:<br>Is the following definition right?<br><br><div class="gmail_quote"><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> "[in ab-initio calculations we] do not make any assumption, such as concerning models and/or fitting parameters."<br></blockquote></div><br><br>I'm saying this because we use, for example, the homogeneous electron gas model for LDA calculations, nonetheless it is still "ab-initio".<br> I think that it's not the use of parameters or models that define a first principles method, but from where those parameters were taken.<br>If your parameters came from experimental values, it's somewhat empirical (or semi-empirical at least), if you used just theoretical values (even if using a simplified model) it can be called ab-initio.<br> <br>Is this right?<br><br>This can even create some discussion if some new functionals can be called "ab-initio" or not...<br><font color="#888888"><br>Leonardo Matheus<br>IF-USP / Brazil</font></div> _______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://www.democritos.it/mailman/listinfo/pw_forum<br></blockquote></div><br><div apple-content-edited="true"> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font face="Helvetica" size="3" style="font: normal normal normal 12px/normal Helvetica; font-size: 12px; ">David E. Farrell</font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Post-Doctoral Fellow</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Department of Materials Science and Engineering</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font face="Helvetica" size="3" style="font: normal normal normal 12px/normal Helvetica; font-size: 12px; ">Northwestern University</font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font face="Helvetica" size="3" style="font: normal normal normal 12px/normal Helvetica; font-size: 12px; ">email: <a href="mailto:d-farrell2@northwestern.edu">d-farrell2@northwestern.edu</a></font></div></div></div></span> </div><br></div></div></body></html>