What is a less restrictive word for "crucial"?

I'm writing my research proposal and I have the following sentence:

The excess charge is considered crucial for the membrane penetration process.

I find the word crucial too strong and I'm searching for another word which is less strong in that context. I thought about required but it doesn't "fit well" with the sentence. Can you think about better word?

Also, I remember that there was a website for synonyms which also had some similar meaning words, but I can't find it. Do you know any of that kind?

If crucial is a bit too "strong" for OP's purposes, I'd have thought the same would apply to vital, necessary, essential, etc., in that they all imply the process simply couldn't take place at all without the charge.

I therefore suggest it's important, which admits of the possibility that the process could take place without the charge, but by implication it would be less efficient, slower, or something. Personally I would also accept central as a "watered down" version of crucial, but I think some people would say that's another "vital" word.

What about necessary?

Adjective
necessary (comparative more necessary, superlative most necessary)
1. needed, required

I like necessary that RiMMER Ψ suggests, an additional choice could be essential.

As for synonyms reference online, dictionaries like Merriam-Webster (http://www.merriam-webster.com/) contain synonyms of the word explained.

Another useful site for synonyms is this one: http://www.synonym.com/

In the context of your sentence, vital works well (in the sense of being essential, necessary, or indispensable).

There are many synonym resources online, but I find the English Synonym Dictionary interesting in its approach.