# Letter to the Editor

## Appeared in the March
1999 issue of *Scientific American*.

**A FRESH ANGLE**
Regarding "Simulating Water and the
Molecules of Life," by Mark Gerstein and Michael Levitt
[November 1998]: the authors write that "the angle formed
between the two sides of the V [of a tetrahedron] is close to
105 degrees--slightly less than the 109.5-degree angle formed
between any two sides of a perfect tetrahedron." Any two
triangular, planar sides of a perfect tetrahedron meet to form
an angle of just over 70 degrees. The V formed by two radii connecting
the geometric center of a regular tetrahedron with two of its
vertices has an angle of 109.5 degrees. These radii represent
the bonds formed between an oxygen nucleus and hydrogen nuclei
in a water molecule; these bonds do form an angle of 105 degrees,
as mentioned previously.

**JOHN W. JOHNSON**

*Santa Barbara, Calif.*

**Gerstein and Levitt reply:**

Johnson is correct that the way we described the geometry
of a water molecule was somewhat imprecise. The exact details
of this geometry and its relation to the tetrahedron are best
expressed not in words but pictorially, as shown at the right.
The bonds in the water molecule correspond to the radii rather
than to the sides of the tetrahedron.

Additional picture showing
tetrahedral geometry in more detail.