I take diazepam for anxiety and 0.5 or 1mg is enough to calm me down (1/4 or 1/2 of 2mg tab). at this subclinical dose, is the effect psychosomatic?

Probably not: Even low dose diazepam generally has anxiolytic effects. However, even at low doses there is a potential for addiction, tolerance & withdrawal with diazepam. So actually it's best for short term use in as low a dose as possible. You should never take this drug without being closely supervised by a licensed physician. I wish you well with your health!