[Chimera-users] How to set the contour level correctly?

Selenschik, Philipp pselensc at uni-muenster.de
Sat Oct 16 06:43:26 PDT 2021

Hi Elaine,

thanks for your quick and helpful reply! I should have been more specific, sorry. My question was about an electron density map of a single particle cryo-EM reconstruction. Concerning the back-calculation, I found an older mailing list entry:


If I understand things correctly: Depending on which normalization is used, the value of the volume dialog to obtain a given (e.g. 3) sigma value can be determined based on the measured SD, RMS and mean by using the relations given above.

Is this what “sdLevel” or “rmsLevel” options of the “Volume” command do?

I would like to show one representative helix of a subunit with its surrounding density (the subunit is fitted into the map).

To do this, I delete all residues except the ones of interest, select the atoms of the helix and use the zone function in the Volume Viewer to display the electron density locally.

But (how) does the radius at which the density is shown relate to/affect the contour level? Are these dependent parameters?

Ideally, I would like to crop the residues of interest at the previously set contour level of the map. Probably there is a more elegant approach to achieve this.

Kind regards


Von: Elaine Meng <meng at cgl.ucsf.edu>
Gesendet: Mittwoch, 13. Oktober 2021 21:29:49
An: Selenschik, Philipp
Cc: chimera-users at cgl.ucsf.edu
Betreff: Re: [Chimera-users] How to set the contour level correctly?

Hi Philipp,
I can't tell if you are talking about electron density like from cryoEM, or a Coulombic electrostatic potential (ESP) map.  Either way, the units shown in the Volume Viewer are not sigma, they are the raw values from the map file.  The "Volume Mean, SD, RMS" tool (or command "measure mapStats") will tell you the mean, standard deviation (SD) from the mean, and the root-mean-square (RMS) deviation from zero but won't affect what is shown in the Volume Viewer.  You'd have to use the stats to back-calculate the contour value that you want, or use command "volume" with "sdLevel" or "rmsLevel" options to do the conversion and set the contour level directly:

If electron density: I don't think there is any universally recommended value.  It depends on the resolution, noise, and other specifics of a given experiment.

If ESP: the Coulombic ESP calculation is mainly for coloring the molecular surface to see relatively positive or negative regions, rather than for display as isopotential contour surfaces.  The values are in kcal/(mol·e) at 298 K.  However, the absolute magnitudes of the values are not that important as you can clearly halve them by doubling the dieletric constant, or double them by halving the dielectric constant, etc.   Thus it is not meant for quantitative calculations, and there is specific recommended contour level for isosurface display.

I hope this helps,
Elaine C. Meng, Ph.D.
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Oct 13, 2021, at 11:31 AM, Selenschik, Philipp via Chimera-users <chimera-users at cgl.ucsf.edu> wrote:
> Hello,
> my apologies if this is a recurring (and potentially trivial) question that does not directly concern Chimeras functionalities.
> I am wondering how to properly set the value of the contour level to visualize a Coulomb potential map.
> When I use the approximation of 1 Da = 1.2 <image003.png>³ to adjust the displayed volume in the Volume viewer dialog almost no secondary structure, but a lot of dust is visible.
> The same problem occurs when I’d like to visualize, for example, an α-helix of a subunit rigid-body fitted to the density. I use the “zone” command to trim the density at a pre-defined 2 Å around the α-helix.
> But is there any general guideline on how to properly set the contour level? I often find values between 1 σ to 3 σ.
> I assume this is the standard deviation of the map. If so, can it be measured via the “Volume Mean, SD, RMS” command?
> Kind regards
> Phil

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