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<br><br><hr id="stopSpelling">From: mark.abraham@anu.edu.au<br>To: gmx-users@gromacs.org<br>Date: Wed, 4 Aug 2010 12:56:54 +1000<br>Subject: Re: [gmx-users] Precision in trajectory file<br><br><br><br>----- Original Message -----<br>From: Inon Sharony <InonShar@TAU.ac.IL><br>Date: Tuesday, August 3, 2010 20:55<br>Subject: [gmx-users] Precision in trajectory file<br>To: "gmx-users@gromacs.org" <gmx-users@gromacs.org>, gmx-developers <gmx-developers@gromacs.org><br><br>> Good afternoon!<br>> <br>> It seems to me that although I'm running a double-precision <br>> installation of GROMACS, the printout to file (of the positions <br>> and velocities) is of much less precision. i.e. the computation <br>> is done on numbers with something like 16 significant digits, <br>> but the last 10 of those are simply truncated and lost (e.g. <br>> positions are given in single-precision as 0.000 nm, and in <br>> double-precision as 0.00000 nm). Since I've already spent <br>> computation time at getting double-precision, I'd like to make <br>> use of all of it -- for my own reasons. I already searched the <br>> manuals, mailing lists and source code for instruction but <br>> didn't find any.<br>> Could you please tell me how I can change the number of digits <br>> printed out (e.g. to the .trr file)? I'm looking for a more <br>> elegant solution than adding a printf line to the source code. <br>> Something along the lines of changing the format of numbers in <br>> the function that prints to .trr .<br><br>You seem confused about something. The .trr file contains a binary representation of the value, in the appropriate precision. There's no nonsense about the number of digits *it* has - it has full single/double-precision binary floating-point numbers, and its precision is not measured in digits.<br><br>Some program that reads those numbers and writes human-readable forms might choose to lose precision, but the underlying precision was always there...<br><br>There's no straightforward way to get large amounts of precision output with GROMACS tools, because only a few of the digits are actually significant. You could hack the code to g_traj or gmxdump to get all the precision if you really want it.<br><br>Mark<br><br>For the next 4.5 beta I added a -fp option to g_traj that will print output in full precision,<br>i.e. exponential format will all relevant decimals for both single and double precision compilation.<br><br>Berk<br><br> </body>
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