(45)The competitive procedure with negotiation should be accompanied by adequate safeguards ensuring observance of the principles of equal treatment and transparency. In particular, contracting authorities should indicate beforehand the minimum requirements which characterise the nature of the procurement and which should not be changed in the negotiations. Award criteria and their weighting should remain stable throughout the entire procedure and should not be subject to negotiations, in order to guarantee equal treatment of all economic operators. Negotiations should aim at improving the tenders so as to allow contracting authorities to buy works, supplies and services perfectly adapted to their specific needs. Negotiations may concern all characteristics of the purchased works, supplies and services including, for instance, quality, quantities, commercial clauses as well as social, environmental and innovative aspects, in so far as they do not constitute minimum requirements. It should be clarified that the minimum requirements to be set by the contracting authority are those conditions and characteristics (particularly physical, functional and legal) that any tender should meet or possess in order to allow the contracting authority to award the contract in accordance with the chosen award criteria. In order to ensure transparency and traceability of the process, all stages should be duly documented. Furthermore, all tenders throughout the procedure should be submitted in writing.