Is Descartes' argument in the Fifth Meditation for the existence of God sound?

Is Descartes' argument in the Fifth Meditation for the existence of God sound?
A sound argument is not just a valid argument: 'it is a valid argument with regard to which there is no more reason to reject the conclusion than to accept the premises'.1 To reject a valid argument on the basis of flawed premises is not a case of belief or disbelief but of rationality or irrationality. Descartes admits that there are two ways of proving the existence of God. Firstly, the causal approach that he takes in the Third Meditation and secondly an a priori approach which he expounds on in the Fifth Meditation. The latter approach is known as the Ontological Argument and involves reasoning independent of any experience. Although with the first method we may need to 'look more closely and investigate more carefully'2 the conclusion is just as certain as the latter method can achieve. However Descartes does say that the second method allows us to acknowledge God 'more easily'3.
THE ARGUMENT
For Descartes the idea of God is one which he finds within him 'just as surely as the idea of any shape or number. And my understanding that it belongs to his nature that he always exists is no less clear and distinct than is the case when I prove of any shape or number that some property belongs to its nature'4
'Existence can no more be separated from the essence of God than the fact that its three angles equal two right angles can be separated from the essence of a triangle'5. Thus it is contradictory to think of God without existence, as it is to think of a triangle without its defining properties. However merely because Descartes thinks of God as existing it does not necessarily entail that God does exist. This is because, as he points out, 'just as I may imagine a winged horse even though no horse has wings, I may be able to attach existence to God even though no God exists'6. However for Descartes what has been established is that a triangle and its defining properties are inseparable. He then applies this line of thought to God and existence - God and existence are inseparable. Therefore God necessarily exists.
A sound argument is not just a valid argument: 'it is a valid argument with regard to which there is no more reason to reject the conclusion than to accept the premises'.1 To reject a valid argument on the basis of flawed premises is not a case of belief or disbelief but of rationality or irrationality. Descartes admits that there are two ways of proving the existence of God. Firstly, the causal approach that he takes in the Third Meditation and secondly an a priori approach which he expounds on in the Fifth Meditation. The latter approach is known as the Ontological Argument and involves reasoning independent of any experience. Although with the first method we may need to 'look more closely and investigate more carefully'2 the conclusion is just as certain as the latter method can achieve. However Descartes does say that the second method allows us to acknowledge God 'more easily'3.
THE ARGUMENT
For Descartes the idea of God is one which he finds within him 'just as surely as the idea of any shape or number. And my understanding that it belongs to his nature that he always exists is no less clear and distinct than is the case when I prove of any shape or number that some property belongs to its nature'4
'Existence can no more be separated from the essence of God than the fact that its three angles equal two right angles can be separated from the essence of a triangle'5. Thus it is contradictory to think of God without existence, as it is to think of a triangle without its defining properties. However merely because Descartes thinks of God as existing it does not necessarily entail that God does exist. This is because, as he points out, 'just as I may imagine a winged horse even though no horse has wings, I may be able to attach existence to God even though no God exists'6. However for Descartes what has been established is that a triangle and its defining properties are inseparable. He then applies this line of thought to God and existence - God and existence are inseparable. Therefore God necessarily exists.
