<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt"><div>Hi all<br><br>I have a problem to understand what a negative isovalue means when I want to visualize the charge density and the electrostatic potential. I am using crystal06 to get these.<br><pre style="font-family: times new roman,new york,times,serif;">I know that the charge density is a function dependent on three variables, namely x,y and z. I can then ask XCrysDen to collect all the points at which the density has a single <br>specific value, the isovalue, and connect them through a three-dimensional plot. I mean rho(x,y,z)=C, where C is the isovalue.</pre>In the isosurface controlling window I get a minimum and maximum grid value. Then I chose a isovalue in this range, and I use the option Render +/-isovalue. Using this last option I get two color surfaces, one in red and other in blue.
The xcrysden <a name="__toc__5">site (http://www.xcrysden.org/doc/isocontrol.html#render+-) gives an example: <br><br>"when displaying the isosurface of a molecular
orbital, then it is a good idea to display both positive and
negative valued isosurface (for example the -0.02/0.02
sqrt(BOHR^3/electrons)). By default the positive isosurface is
rendered in red, while negative in blue."<br><br>The problem comes when I wanto to interpret it for electronic density, which is given in electrons/BHOR</a><a name="__toc__5">^3 </a><a name="__toc__5">by crystal06 and this quantity should be POSITIVE. First, the isovalue is the square root of the inverse of the electronic density (at least by the units it seems)</a><a name="__toc__5">, as is shown in the xcrysden site, so what a negative isovalue means????? Does it mean less quantity of electrons????.<br><br>On the other hand, when instead of surfaces we have planes, we have to specify the lowest and the higest render value. This ones are shown in the thermometer which has the label DELTA n(r), Does it means a difference between two isovalues or a difference between other quantities to get the isovalues???? and again here the question </a><a name="__toc__5"> returns </a><a name="__toc__5">about negative isovalues, what does a negative isovalue
means in the thermometer???? <br><br>In the section for property-planes display parameters we have to select a scale function (http://www.xcrysden.org/doc/plane1control.html#__toc__5). Here is recomended:<br><br>"For example for
charge density plots the logarithmic scale is usually more
appropriate as linear, whereas for charge density difference maps
the linear scale is usually better"<br><br><br>Why for the same property, just to see the difference maps we have to change the scale function ??????</a><a name="display"></a> <a name="display-book"></a>
<h3><a name="__toc__6"></a></h3><a name="__toc__5">Finally, for the electrostatic potential we have the same isosurface controlling window. So, I guess the isosurfaces color ( blue and red) have the right meaning for positive and negative charges, isn't it?<br><br><br>Thanks before hand to help me to clarify my doubts.<br><br></a><pre><br>Regards, Ariadna</pre><a name="__toc__5"><br> </a></div>
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