Probably a density volume, such as

https://i.stack.imgur.com/V9Nsa.png

Depending on your system, the volume can be easy or hard to read. Maybe taking the slices of it, which results in regular FES diagram, would be useful to illustrate things.

Sun Tiedong

Thank you seems reasonable.

Which tool could produce such density volume graph?

Sayyed Jalil Mahdizadeh

VMD can do this with GAUSSIAN .cube file.

http://www.ks.uiuc.edu/Training/Tutorials/cpmd-vmd.pdf

I'm sure many other softwares can do the same job (for example, in CFD).

ParaView seems one of them.

https://discourse.paraview.org/t/volume-rendering-of-scalar-field-with-selective-opacity/329

You can use metadynminer3d.

http://metadynamics.cz/metadynminer3d/