<div dir="ltr"><div>Dear Mark and Erik,<br><br></div><div>Just a quick note to say thanks a lot for your advice. I was able to get a much clearer picture of the internals of gromacs, because of it.<br><br></div><div>Best regards,<br>
<br></div><div>James<br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On 3 February 2014 10:28, Mark Abraham <span dir="ltr"><<a href="mailto:mark.j.abraham@gmail.com" target="_blank">mark.j.abraham@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Yeah, that's what I meant. Multiply thing by -1 or take some other thing to the power -1, it's all the same right? :-P<span class="HOEnZb"><font color="#888888"><div>
<br></div><div>Mark</div></font></span></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><br><div class="gmail_quote">
On Sun, Feb 2, 2014 at 8:51 PM, Erik Lindahl <span dir="ltr"><<a href="mailto:erik.lindahl@scilifelab.se" target="_blank">erik.lindahl@scilifelab.se</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi,<br>
<div><br>
On 02 Feb 2014, at 20:11, Mark Abraham <<a href="mailto:mark.j.abraham@gmail.com" target="_blank">mark.j.abraham@gmail.com</a>> wrote:<br>
><br>
> Do take care - components of fscal are not actually the scalar force, but the negative of it (because mostly it saves a flop).<br>
<br>
</div>For the record, it can be even worse than this. In many (most) places we need to multply the force with the interparticle vector to get the vectorial force. However, it would be expensive (and stupid) computationall to normalize this vector when we already have 1/r available. Thus, fscal is often the scalar force *divided by r*, so we can then multiply it by the non-normalized interparticle vector.<br>
<div><div><br>
Cheers,<br>
<br>
Erik<br>
<br>
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