private boolean evaluateChargeClause(String str) {
		double x = parseMathExpression(str);
		if (Double.isNaN(x))
			return false;
		int n = model.getNumberOfParticles();
		if (n <= 0)
			return true;
		float c = (float) x;
		for (int k = 0; k < n; k++) {
			Particle p = model.getParticle(k);
			if (p.isSelected())
				p.setCharge(c);
		}
		if (model instanceof AtomicModel)
			((AtomicModel) model).checkCharges();
		view.repaint();
		notifyChange();
		return true;
	}