The most common (and preferred) type of displayed equation in The APS Journals is a single-line equation, with an equation number on the same line. Try to set as many equations as you can in this way.
Equations are now normally set centered in the column width for APS style
with REVTeX . Setting the equation number is taken care of by
REVTeX ---the number will be set below the equation if necessary.
Breaking the equation into multiline format may be necessary for very long
equations. The eqnarray environment is used for this purpose.
Do not use plain array for this purpose.
Break at a sign of relation or an operator sign; the sign (e.g., =, +,
) begins the next line of the equation. Specify (using
&&) a proper indent or alignment of the line following the
break; e.g., +, -, 
should line up to the right of an = above, not
directly under it. A
multiline equation centers as a unit. Use a separate equation or
eqnarray environment (\begin{#1}-\end{#1} command
pair) for each single-line equation or multiline equation. Short
displayed equations that can appear together on a single line may be placed
in one equation environment.
In galley style, if an equation needs to be broken into many lines, it
is suggested that it be
set in a wide column for ease of reading, using the \widetext
command. The author should return to \narrowtext as soon as possible
after one or more very long equations, but short pieces of narrow text
and/or math between nearly contiguous wide sections should be left wide and
incorporated into the surrounding wide sections.
In apssamp.tex, we have illustrated how to obtain each of the above.