If you have bank of america you can link your bofa and merrill accounts. They even give you bonuses on your credit cards of +25/50/75% cash back for balances over 25/50/100k in assets between both.
They increase the cash back by 75% of whatever it already was. So if you were getting 3% back (currently, for their Cash Rewards card, that can be 1 of the following categories: gas, online shopping, dining, travel, drug stores, or home improvement/furnishings), you'd now be getting 5.25% back.
Which is a complete rip-off. You could easily get 2% interest on that $100k you're leaving with them by just leaving it with a better bank. For that to be worth it, you'd need to get at least $2k cash back on that credit card. At 5.25% (which is only for that 1 category you chose!) you'd need to burn through $38k/year... i.e., you gotta $100 a day on that card.
Except even if you somehow were planning to spend $100/day on that one lucky category, your cash back would be an order of magnitude lower, because they'd only give you that cash back on the first $2,500 in purchase, i.e. you could only earn $131 at most. So you're losing out on at least $2,000 to earn at most $131. Terrific deal!
The more optimal method is to leave 1-3 months worth of expenses as cash and put the rest of the money in something like VTI. You don't want a market crash to erase half of your emergency fund at the exact moment you need it.
Emergency funds should be in FDIC insured savings accounts, that pay at least 50 basis points less than the fed funds rate.
Also, you should have a stash of cash you can find at home in case of power outage or network loss and you can’t get money from the bank. And guns and canned food and clean water.
You can still get 2% interest on that buy buying treasury ETFs, or 1.56% currently by asking them to open a preferred deposit account for you, which is essentially a savings account with a high interest rate. You dont have to keep it in cash.