We study
S′+, of all
aD′ such that
(a*σ,)≥0 for every
C∞0, where
*σ denotes the twisted convolution. We prove that certain boundedness for
aS′+ are completely determined of the behaviour for
a at origin, for example that
aS′, and that if
a(0)<∞, then
aL2∩L∞. We use the results in order to determine wether positive pseudo-differential operators belong to certain Schatten-casses or not.