Opening Bid:   Two Diamonds

Long ago we all played strong two bids, but in the 1960s we started playing weak twos, which were hated by the old-time players who didn't know how to defend against such blasphemy.  Well, today we all know what to do over an opponent's weak diamond call.  Although the weak two spade call is still pretty effective, the weak two diamond bid simply makes bidding easier for the opponents because you have given them a look at your hand while there is still lots of bidding room.  It's not a nearly as preemptive as two spades.

There's a better use for the bid.
    An opening bid of 2 shows an opening hand with at least a 5-card diamond suit and denies a 4-card major.  It is used when you do not want to rebid 1NT after a response by your partner.

Negative Double?
Because the opener does not have a 4-card major you should not use negative doubles.  If an opponent overcalls your opening bid of 2 and your partner doubles, he means it for penalty, so PASS!

Invitational Responses
A 2-level response in either notrump or a major is highly invitational but not forcing.  The opener should strive to find a reason to bid but is not obligated do so.

Forcing Responses
All 3-level suit responses are natural and forcing. Responder has an opening hand and wants to be in game.

I can't tell you that this is a popular agreement as there are many uses for an opening 2 call, but it is useful with a strong club bidding system, and is similar to the 2 call, making it consistent with that bid and easy to remember.