The **measure** command performs various calculations
and sends results to the **Log**.
Possible values of *property*:

**area**– report the area of an existing surface (sum of surface triangles)**blob**– measure surface area, enclosed volume, and dimensions of a disconnected surface blob**buriedarea**– calculate solvent-accessible surface area buried between two sets of atoms**center**– calculate center of mass of map and/or atoms**convexity**– calculate convexity at each surface vertex, color the surface accordingly**inertia**– calculate inertia ellipsoid**length**– sum bond lengths**mapstats**– report volume data min, max, mean, SD, and RMSD values**motion**– show changes in surface position by drawing lines**sasa**– calculate solvent-accessible surface area**symmetry**– check map for certain symmetries in standard orientations**volume**– report the volume enclosed by an existing surface

See also: **interfaces**,
**surface**,
**surface zone**,
VDW radii,
measurements

Report the total surface area of an existing surface model, computed as the sum of the areas of its triangles. The•includeMaskedoption controls whether to include parts of the surface that have been hidden, such as withsurface dustorsurface zone(clipping does not affect the measurement, however). See also:measure sasa

Report measurements for the blob (disconnected surface part) in the specified surface model containing•triangleNumberN. The surface-model specification cannot be blank. Although not generally known in advance, the triangle number is included in the command echoed to theLogwhen thepick blobmouse mode is used, so that the action could be replicated in a script. Measurements include:Blob color is left unchanged unless a

volumeenclosed in the blobareaof its triangulated surface- if
reportSizeistrue(default), thesizein dimensions from longest to shortest of the bounding box aligned with principal axes (details...)coloris given. Theoutlineoption shows the bounding box as an outline in the specifiedoutlineColor(defaultlime). See also:

Measure and Color Blobs

Calculate the solvent-accessible surface (SAS) area buried between two sets of atoms, defined as:•½ (wheresasa1+sasa2–sasa12)sasa1is the area of the SAS enclosing the atoms inatom-spec1,sasa2is the area of the SAS enclosing the atoms inatom-spec2, andsasa12is the area of the SAS enclosing both sets of atoms together. The sets of atoms should be specified with care; they should not overlap, and solvent, ions, and ligand residues are not excluded automatically. Unspecified atoms are ignored. The defaultprobeRadiusradfor calculating each SAS is1.4Å, often used to approximate a water molecule. Residues with at leastcutoffAreaareaburied (default1.0Å^{2}) can be:

- listed along with their buried areas in the
Logusing thelistResiduesoption- selected with the
selectoption- colored using
colorcolor-specThe buried area of a residue is its SAS area in the individual set minus that in the combined set. Examples:

measure buriedarea (/c & protein) with (/d & protein)

– calculate buried surface area between theproteinparts only of chains C and D

measure buried ligand with protein list T sel T

– select and list residues with ≥ 1.0 Å^{2}area buried betweenligandandprotein

Calculate the center of mass of each density map and/or set of atoms in•spec. Map centers are reported in grid indices, atomic centers of mass in the atomic coordinate system. Theleveloption indicates using only map regions abovecontour-level. Ifmarkistrue, a marker will be placed at at each computed center, with radiusmarker-radius(default based on the contents ofspec) and color (default#b4b4b4). The marker model is opened as number

model-number(default next unused number) with namemodel-name(default based on the contents ofspec). See also:cofr

Color a surface based on the convexity at each vertex, calculated as 2π minus the cone-angle spanned by the triangles incident at the vertex. Convexity values are smoothed by averaging with neighboring (edge-connected) vertices for a specified number of iterations (default•5). Smoothing is generally recommended, given that this definition of convexity is nonstandard and the unsmoothed values depend strongly on the triangulation: vertices surrounded by large triangles on a smooth surface will have sharper cone angles than vertices surrounded by small triangles. (Normalizing by triangle areas does not help because the patch around a vertex is often irregular in shape.) The surface vertex positions, normals, convexity values, and triangles can be saved to a text file withwriteSurfaceData, wherefilenamecan be a pathname including the directory location.The remaining options specify coloring. The

patchesoption randomly assigns colors to contiguous patches of vertices with convexity values above theconvexity-threshold. Otherwise (patchesnot used), the surface will be colored by the convexity value per vertex, withpalette-optionsas described forcolor, except with defaults:

Unsmoothed values typically give mottled coloring. See also:palette cyan-gray-maroon range -1,1mlp,color,bumps

Calculate the inertia ellipsoid for•atom-spec, which could include atoms and/or surfaces. Atoms are mass-weighted; surfaces are treated as thin shells with mass proportional to surface area (details...). If both atoms and surfaces are specified, separate ellipsoids are calculated (a combined calculation cannot be performed). Principal axes, lengths, moments, and center are reported for each ellipsoid, using the model coordinate system of the first atom or surface specified to define it. The vectors v1, v2, and v3 are the principal axes (longest to shortest). The lengths a, b, c are half-diameters along axes v1, v2, and v3, respectively. The moments r1, r2, and r3 are calculated as (inertia/mass)^{½}about axes v1, v2, and v3, respectively. They can be considered effective radii; placing all of the mass at that distance from the center would reproduce the moment of inertia calculated for the structure around that axis.The

perChainoption indicates whether to calculate a separate ellipsoid for each chain inatom-spec. IfshowEllipsoidistrue(default), the ellipsoid(s) will be opened as a surface model withmodelIdmodel-number(default the next unused number), containing multiple submodels if there are multiple ellipsoids. Thereplace trueoption allows replacing an existing model when the specifiedmodel-numberis already in use. If ellipsoidcoloris not specified, each ellipsoid will be colored to match the first atom or surface in its calculation.Another way to generate a low-resolution representation of an atomic structure is with

molmap.

Sum the lengths of all bonds between specified atoms (markers); primarily used to measure the length of traced paths of markers.•

Report the minimum value, maximum value, mean, standard deviation (SD) from the mean, and the root-mean-square (RMS) deviation from zero for the specified volume data models, if any, otherwise all such models. The•stepandsubregionoptions can be used to limit the calculation to a subsample or spatial subregion of the data. The step size must be an integer; 1 indicates all data points (default), 2 indicates every other data point, 3 every third point,etc.If a single number is supplied, it is used along all three axes; if three numbers are supplied (separated by commas but not spaces), they are used along the X, Y, and Z axes, respectively. A subregion can be specified by:

- grid indices
i1–i2along the X axis,j1–j2along the Y axis, andk1–k2along the Z axis. Grid indices must be integers separated by commas but not spaces.- the word
all, indicating the full extent of the data (default) rather than a subregion

Draw “prickles” to show the change in position between a surface and a volume (map) isosurface, for example, between time steps of a volume series. Prickles are line segments drawn perpendicular to the surface. They are extended from the vertices of•surf-modelin increments of map grid units (using the smallest spacing along the three axes, if they differ) until they intersect with the map isosurface or reachstepsMgrid units in length (default10). Prickles are shown in the specifiedcolor(defaultlime) and can be amplified or shrunken in length by a

scalefactorf. If a model number is specified withpricklesModelN, the prickles will be added as a submodel ofN. If a model number is not specified, the new model will be a submodel ofsurf-model.

Calculate the area of a solvent-accessible surface (SAS) enclosing the atoms in•atom-spec1and report the total value in theLog. ThesetAttributeoption specifies whether to assign the values per atom and residue as attributes namedarea(defaulttrue). Thesumoption can be used to report the area contribution from some subset of the atoms (given asatom-spec2). The calculated SAS is not displayed. The atoms should be specified with care; solvent, ions, and ligand residues are not excluded automatically. Unspecified atoms are ignored, as are atoms inatom-spec2that are not also inatom-spec1. The defaultprobeRadiusradfor calculating the SAS is1.4Å, often used to approximate a water molecule. Example:See also:measure sasa #1/a & protein sum :phe,tyr,trp

– calculate the SAS of theproteinin model #1 chain A and report both the total area and the collective contribution from phenylalanine, tyrosine, and tryptophan residuesmeasure area

Check each specified volume data model (map) for cyclic, dihedral, tetrahedral, octahedral, and icosahedral symmetries in standard coordinate systems. Helical symmetry can be considered if approximate parameters are supplied. The symmetry assignment can be used by other commands such as•sym,molmap, andfitmap, and is included in Chimera map format. For direct assignment of a specified symmetry, seevolume symmetry.If the correlation of the map with itself after symmetry transformation is at least

mincorr(default0.99), the detected type of symmetry will be reported, and ifsetis true (default), assigned to the map in ChimeraX. The correlation calculation uses only map points with values above the displayed contour level; if the number of such points exceedsmaxpts(default10,000), a random sample ofmaxptsis chosen from them and used. Values in the first copy of the map are compared with the superimposed (interpolated) values in the rotated copy of the map.Center of point symmetry is considered only at the following:

For cyclic and dihedral symmetry, rotation is considered only about the Z axis, and for dihedral symmetry, flipping symmetry only about the X or Y axes. Cyclic (C

- the grid point nearest the average indices of grid points with values above the displayed contour level. The map's lowest contour level in
Volume Vieweris used.- one or two grid points based on the overall map dimensions: only the midpoint along axes with odd numbers of points, and along axes with even numbers of points, those on either side of the midpoint. Rather than all possible combinations for axes with even numbers of points, only the two points with all indices lower or all higher are evaluated.
n) symmetry is checked for ordernup tonMax, default8. If more than one Cnsymmetry meets the criterion, those for which a higher multiple is also found are discarded, and of the remaining, the one with the highest correlation is assigned. For example, ifn= 2, 3, 6, and 7 were to meet the criterion, 6-fold would override 2- and 3-fold, and 6-fold or 7-fold symmetry, whichever gave the highest correlation, would be assigned. Tetrahedral symmetry is considered in two orientations:

- 2-folds along X, Y, and Z, with a 3-fold along axis (1,1,1)
- 3-fold along Z, with a second 3-fold in the YZ plane such that rotation about the X axis by ~110° is a symmetry operation (EMAN convention)
Icosahedral symmetries are only considered in eight orientations:

- 222 – with two-fold symmetry axes along the X, Y, and Z axes
- 2n5 – with two-fold symmetry along X and 5-fold along Z
- n25 – with two-fold symmetry along Y and 5-fold along Z
- 2n3 – with two-fold symmetry along X and 3-fold along Z
- 222r – same as 222 except rotated 90° about Z
- 2n5r – same as 2n5 except rotated 180° about Y
- n25r – same as n25 except rotated 180° about X
- 2n3r – same as 2n3 except rotated 180° about Y
The

helixoption specifies looking for helical symmetry with approximaterise(in physical units of distance, typically Å) andangle(degrees) per asymmetric unit. If this option is used, the other types of symmetry are not considered except for combined helical and cyclic symmetry (for example, EMD-1757, approximately 42 Å rise and 21° twist per subunit). Helical symmetry is infinite, but the number of copies to place when considering that symmetry,n, is necessarily finite. If not given,nwill be determined by dividing the apparent length of the helix in the map by the rise and rounding to the nearest positive integer. Theoptkeyword indicates optimizing the fit of the map copies to itself to identify more accurate helical parameters.

Report the volume enclosed by an existing surface model. TheincludeMaskedoption controls whether to include parts of the surface that have been hidden, such as withsurface dustorsurface zone(clipping does not affect the measurement, however).

**Inertia calculation**.

The command **measure inertia**
computes the moments of inertia of a set of atoms as in classical mechanics:

II is a 3x3 matrix with indices j and k (j=1,2,3 and k=1,2,3). Each matrix element is a sum over atoms, where m_{jk}= Σ_{i}(m_{i}(δ_{jk}|x_{i}|^{2}– x_{i,j}x_{i,k}))

- For atoms, we show the surface of a uniform-density solid ellipsoid that has the same principal axes and moments as the atoms.
- For surfaces, we show an ellipsoidal surface that as a thin shell has the same axes and moments as the measured surface.

UCSF Resource for Biocomputing, Visualization, and Informatics / May 2020