for the President to perform his constitutional duties, it is imperative that he receive candid and unfettered advice and that free and open discussions and deliberations occur among his advisers and between those advisers and others within and outside the Executive Branch. Presidents would not be able to fulfill their responsibilities if their advisers on fear of being commanded to Capitol Hill to testify or having their documents produced to Congress were reluctant to communicate openly and honestly in the course of rendering advice and reaching decisions. These confidentiality interests are especially strong in situations like the present controversy, where the inquiry seeks information relating to the President's powers to appoint and remove U.S. Attorneys — authority granted exclusively to the President by the Constitution.