A soft-spoken psychiatrist, driven into a murderous rage when an affair with a female patient went sour, solicited another patient to help him get a gun and silencer to kill her and five others, authorities in New York said yesterday.

Richard Karpf, 50, was held without bail yesterday after being charged with asking a patient at his Long Island psychiatric practice to set him up with a gun dealer, according to Fred Klein, a Nassau County assistant district attorney.

The patient notified police and an undercover officer posing as an unlicensed gun dealer met with Karpf and offered to sell him a .22-caliber semi-automatic pistol, four ammo clips, a box of 50 rounds and a threaded silencer that could be screwed onto the muzzle of the gun.

"He wanted to shoot the people point-blank in the head and in the heart," Klein said.

Karpf had aspirations of mass murder, according Klein.

He wanted to commit the murders "all in one place," Klein said. "I don't believe it was going to be in his office."

Karpf wanted to chop up the bodies, put them in plastic bags, "take all this out on a boat in the Atlantic and dispose of the evidence in the ocean," according to Klein.

One of Karpf's patients - a woman with whom he allegedly had an affair - was notified by cops she was a possible target, according to her lawyer, Peter DeFilippis.

The woman, who had been treated by Karpf for about a year, confronted him Tuesday, after which "he became quite irate and chased her," said DeFilippis, adding that his client wished to remain anonymous.

The woman reported Karpf to the police and was later notified by homicide detectives that Karpf "made statements indicating he intended to take care of a problem and . . . strongly intimated that the problem was her," DeFilippis said.

Karpf, who received his M.D. in 1980 from a university in Guadalajara, Mexico, was arrested Wednesday afternoon in the parking lot of a Long Island Home Depot.

He had $1,600 and an empty bag to carry off the weaponry, Klein said.

Two hours before he was arrested, Karpf walked into Nassau County police headquarters - which he could see from his office window, a block away - and picked up an application for a gun permit, said Detective Lt. Dennis Farrell, the Homicide Squad commander.

A patient of Karpf's, who arrived at his office for an appointment yesterday, blanched when she was told of her shrink's plan to kill at least one patient, but called him a "good listener" and a "nice guy."

"I'm thinking he was not taking care of himself if he went off the deep end like that," she said. "If he was talking about this, the likelihood is he was reaching out for help."

Karpf was arraigned yesterday and pleaded not guilty to two counts of second-degree weapons possession and a third-degree weapons charge, all felonies.

As he was into court with his hands cuffed behind his back, Karpf said, "I deny all of that, none of that is true . . . It's a mistake."

Karpf's bail application was rejected and he was taken to Nassau County Jail.

He faces up to 15 years in prison on the most serious charges and is due back in court Monday.

Authorities said they are reviewing video and audiotapes and expect to present the case to a grand jury for a possible upgrade of charges, including conspiracy to commit murder.

Karpf's father, a retired real-estate lawyer, was present in court but kept mum about his son's predicament.

A woman answering the phone at Karpf's parents' house said Karpf was buying the gun to protect himself from his psychiatric patients.

"Oh, no, he wouldn't kill anybody," the woman said. "People with mental problems are walking into your office - in other words, you're a sitting duck in that office."

Neighbors at Karpf's posh Long Island apartment complex described him as mild-mannered and quiet.

"He's a polite man, he minds his own business and doesn't bother anyone," Vivian Jacobs said. "He has never caused any trouble at all."